Correct the Windows cross-compiling commands (#13071)

### Summary

During the cross-compilation using Clang on Ubuntu for Windows, there
are several commands that are executed on the host and they are failing.

Add the guarding condition which checks for `CMAKE_CROSSCOMPILING` and
picks the Linux alternative.

### Test plan

<details>
<summary>1. Create a docker image for cross compilation</summary>

```
ARG GROUP_ID=1000
ARG USER_ID=1000
ARG USER_NAME=docker-user

FROM amd64/ubuntu:24.04
# https://bugs.launchpad.net/cloud-images/+bug/2005129
RUN userdel -r ubuntu

ENV DEBIAN_FRONTEND=noninteractive

ARG GROUP_ID
ARG USER_ID
ARG USER_NAME

RUN apt-get -y update \
    && apt-get install --no-install-recommends -y \
    ccache \
    curl \
    lsb-release \
    wget \
    software-properties-common \
    gnupg \
    ca-certificates \
    build-essential \
    git \
    make \
    ninja-build \
    patch \
    && rm -rf /var/lib/apt/lists/*

RUN wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | apt-key add - \
    && wget https://apt.llvm.org/llvm.sh \
    && chmod +x llvm.sh \
    && ./llvm.sh 19 all \
    && apt update && apt install -y \
        clang-19 \
        lld-19 \
        llvm-19 \
    && rm -rf /var/lib/apt/lists/*

RUN update-alternatives --install /usr/bin/clang-cl clang-cl /usr/bin/clang-cl-19 60 \
    && update-alternatives --install /usr/bin/clang clang /usr/bin/clang-19 60 \
    && update-alternatives --install /usr/bin/clang++ clang++ /usr/bin/clang++-19 60 \
    && update-alternatives --install /usr/bin/llvm-rc llvm-rc /usr/bin/llvm-rc-19 60 \
    && update-alternatives --install /usr/bin/llvm-mt llvm-mt /usr/bin/llvm-mt-19 60 \
    && update-alternatives --install /usr/bin/lld-link lld-link /usr/bin/lld-link-19 60 \
    && update-alternatives --install /usr/bin/lldb lldb /usr/bin/lldb-19 60 \
    && update-alternatives --install /usr/bin/llvm-lib llvm-lib /usr/bin/llvm-lib-19 60 \
    && update-alternatives --install /usr/bin/llvm-ar llvm-ar /usr/bin/llvm-ar-19 60 \
    && update-alternatives --install /usr/bin/llvm-ranlib llvm-ranlib /usr/bin/llvm-ranlib-19 60 \
    && update-alternatives --install /usr/bin/llvm-nm llvm-nm /usr/bin/llvm-nm-19 60 \
    && update-alternatives --install /usr/bin/llvm-objdump llvm-objdump /usr/bin/llvm-objdump-19 60 \
    && update-alternatives --install /usr/bin/llvm-objcopy llvm-objcopy /usr/bin/llvm-objcopy-19 60 \
    && update-alternatives --install /usr/bin/llvm-strip llvm-strip /usr/bin/llvm-strip-19 60 \
    && update-alternatives --install /usr/bin/cc cc /usr/bin/clang-19 100 \
    && update-alternatives --install /usr/bin/c++ c++ /usr/bin/clang++-19 100 \
    && update-alternatives --install /usr/bin/ld ld /usr/bin/ld.lld-19 100

# -----------------------------------------------------------------------------
# user
# -----------------------------------------------------------------------------
RUN groupadd --gid $GROUP_ID docker-user \
    && useradd --uid $USER_ID --gid docker-user --create-home $USER_NAME

USER $USER_NAME

# -----------------------------------------------------------------------------
# python uv
# -----------------------------------------------------------------------------
RUN curl -LsSf https://astral.sh/uv/0.7.17/install.sh | sh

# -----------------------------------------------------------------------------
# rust, x86_64-pc-windows-msvc target
# -----------------------------------------------------------------------------
RUN curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- --profile minimal -y

# -----------------------------------------------------------------------------
# xwin
# -----------------------------------------------------------------------------
RUN ~/.cargo/bin/cargo install xwin cargo-cache \
    && ~/.cargo/bin/cargo-cache -a \
    && ~/.cargo/bin/xwin --cache-dir /tmp/xwin-cache --accept-license --variant desktop --arch x86_64 --include-atl \
    splat --preserve-ms-arch-notation --include-debug-libs --output ~/.xwin/x86_64 \
    && rm -rf /tmp/xwin-cache
```

</details>


<details>
<summary>2. Create cmake dir.  </summary>

For some reason I had to disable AVX instructions to get it working, it
is not the intention, the intention here is to use a window toolchain

```
cmake -G Ninja -B build-win \
    -DCMAKE_BUILD_TYPE=Release \
    -DCMAKE_SYSTEM_NAME=Windows \
    -DCMAKE_SYSTEM_VERSION=10.0 \
    -DCMAKE_SYSTEM_PROCESSOR=AMD64 \
    -DCMAKE_C_COMPILER=clang-cl \
    -DCMAKE_CXX_COMPILER=clang-cl \
    -DCMAKE_ASM_COMPILER=clang-cl \
    -DCMAKE_RC_COMPILER=llvm-rc \
    -DCMAKE_LINKER=lld-link \
    -DCMAKE_C_COMPILER_TARGET=x86_64-pc-windows-msvc \
    -DCMAKE_CXX_COMPILER_TARGET=x86_64-pc-windows-msvc \
    -DCMAKE_ASM_COMPILER_TARGET=x86_64-pc-windows-msvc \
    -DCMAKE_SYSROOT=/home/docker-user/.xwin/x86_64 \
    -DCMAKE_FIND_ROOT_PATH=/home/docker-user/.xwin/x86_64 \
    -DCMAKE_FIND_ROOT_PATH_MODE_PROGRAM=NEVER \
    -DCMAKE_FIND_ROOT_PATH_MODE_LIBRARY=ONLY \
    -DCMAKE_FIND_ROOT_PATH_MODE_INCLUDE=ONLY \
    -DCMAKE_CXX_FLAGS="/imsvc /home/docker-user/.xwin/x86_64/crt/include /imsvc /home/docker-user/.xwin/x86_64/sdk/include/ucrt /imsvc /home/docker-user/.xwin/x86_64/sdk/include/um /imsvc /home/docker-user/.xwin/x86_64/sdk/include/shared -Wno-unknown-argument" \
    -DCMAKE_C_FLAGS="/imsvc /home/docker-user/.xwin/x86_64/crt/include /imsvc /home/docker-user/.xwin/x86_64/sdk/include/ucrt /imsvc /home/docker-user/.xwin/x86_64/sdk/include/um /imsvc /home/docker-user/.xwin/x86_64/sdk/include/shared -Wno-unknown-argument" \
    -DCMAKE_EXE_LINKER_FLAGS="/libpath:/home/docker-user/.xwin/x86_64/crt/lib/x64 /libpath:/home/docker-user/.xwin/x86_64/sdk/lib/um/x64 /libpath:/home/docker-user/.xwin/x86_64/sdk/lib/ucrt/x64" \
    -DCMAKE_SHARED_LINKER_FLAGS="/libpath:/home/docker-user/.xwin/x86_64/crt/lib/x64 /libpath:/home/docker-user/.xwin/x86_64/sdk/lib/um/x64 /libpath:/home/docker-user/.xwin/x86_64/sdk/lib/ucrt/x64" \
    -DGFLAGS_INTTYPES_FORMAT=VC7 \
    -DEXECUTORCH_BUILD_XNNPACK=ON \
    -DXNNPACK_ENABLE_ASSEMBLY=OFF \
    -DXNNPACK_BUILD_TESTS=OFF \
    -DXNNPACK_BUILD_BENCHMARKS=OFF \
    -DXNNPACK_ENABLE_AVX512F=OFF \
    -DXNNPACK_ENABLE_AVX512SKX=OFF \
    -DXNNPACK_ENABLE_AVX512VBMI=OFF \
    -DXNNPACK_ENABLE_AVX512VNNI=OFF \
    -DXNNPACK_ENABLE_AVX512VNNIGFNI=OFF \
    -DXNNPACK_ENABLE_AVX512AMX=OFF \
    -DXNNPACK_ENABLE_AVX512FP16=OFF \
    --fresh
```
</details>

<details>
<summary>3. Build </summary>

```
cmake --build build-win --config Release --target xnnpack_schema
```
</details>

The error will have 
```
FAILED: schema/include/executorch/backends/xnnpack/serialization/schema_generated.h /mnt/executorch/build-win/schema/include/executorch/backends/xnnpack/serialization/schema_generated.h
cd /mnt/executorch && /mnt/executorch/build-win/third-party/flatbuffers_external_project/bin/flatc --cpp --cpp-std c++11 --scoped-enums -o /mnt/executorch/build-win/schema/include/executorch/backends/xnnpack/serialization backends/xnnpack/serialization/runtime_schema.fbs && powershell -Command "Move-Item -Path /mnt/executorch/build-win/schema/include/executorch/backends/xnnpack/serialization/runtime_schema_generated.h -Destination /mnt/executorch/build-win/schema/include/executorch/backends/xnnpack/serialization/schema_generated.h"
/bin/sh: 1: powershell: not found
ninja: build stopped: subcommand failed.
```

Author: ykhrustalev

backends/xnnpack/CMakeLists.txt

@@ -59,7 +59,7 @@ foreach(fbs_file ${_xnnpack_schema__srcs})
   )
 endforeach()
 
-if(WIN32)
+if(WIN32 AND NOT CMAKE_CROSSCOMPILING)
   set(MV_COMMAND
       powershell -Command
       "Move-Item -Path ${_xnnpack_flatbuffer__outputs} -Destination ${_xnnpack_schema__outputs}"

third-party/CMakeLists.txt

@@ -49,7 +49,7 @@ ExternalProject_Add(
 ExternalProject_Get_Property(flatbuffers_external_project INSTALL_DIR)
 add_executable(flatc IMPORTED GLOBAL)
 add_dependencies(flatc flatbuffers_external_project)
-if(WIN32)
+if(WIN32 AND NOT CMAKE_CROSSCOMPILING)
   # flatbuffers does not use CMAKE_BUILD_TYPE. Internally, the build forces Release
   # config, but from CMake's perspective the build type is always Debug.
   set_target_properties(flatc PROPERTIES IMPORTED_LOCATION ${INSTALL_DIR}/bin/flatc.exe)
@@ -101,7 +101,7 @@ file(REMOVE_RECURSE ${PROJECT_SOURCE_DIR}/third-party/flatcc/lib)
 ExternalProject_Get_Property(flatcc_external_project INSTALL_DIR)
 add_executable(flatcc_cli IMPORTED GLOBAL)
 add_dependencies(flatcc_cli flatcc_external_project)
-if(WIN32)
+if(WIN32 AND NOT CMAKE_CROSSCOMPILING)
   set_target_properties(flatcc_cli PROPERTIES IMPORTED_LOCATION ${INSTALL_DIR}/bin/flatcc.exe)
 else()
   set_target_properties(flatcc_cli PROPERTIES IMPORTED_LOCATION ${INSTALL_DIR}/bin/flatcc)