Add support for CLANGARM64 target (git-for-windows#3916)

**This requires an ARM64-machine with Windows 11 installed (which
supports x64 emulation for MSYS2)**

### The main idea

- Use the main MSYS2/git-sdk-64 setup, which works on Windows 11 on ARM
thanks to x64-emulation
- Configure the official `clangarm64` MSYS2 repo
- Install `mingw-w64-clang-aarch64-toolchain` which contains the
ARM64-native Clang compiler

Author: dscho

.github/workflows/clangarm64-build.yml

@@ -0,0 +1,25 @@
+name: CLANG build ARM64
+
+on:
+  workflow_dispatch:
+
+defaults:
+  run:
+    shell: bash
+
+jobs:
+  clang-build:
+    runs-on: [Windows, ARM64]
+    env:
+      NO_PERL: 1
+    steps:
+      - uses: actions/checkout@v3
+      - uses: git-for-windows/setup-git-for-windows-sdk@v1
+        with:
+          flavor: makepkg-git
+          architecture: aarch64
+          # This assumes that the job is running on a self-hosted runner,
+          # in which case we need to cleanup SDK files.
+          cleanup: true
+      - name: Build Git CLANGARM64
+        run: make -j`nproc`

config.mak.uname

@@ -711,6 +711,10 @@ else
 			prefix = /mingw64
 			HOST_CPU = x86_64
 			BASIC_LDFLAGS += -Wl,--pic-executable,-e,mainCRTStartup
+		else ifeq (CLANGARM64,$(MSYSTEM))
+			prefix = /clangarm64
+			HOST_CPU = aarch64
+			BASIC_LDFLAGS += -Wl,--pic-executable,-e,mainCRTStartup
 		else
 			COMPAT_CFLAGS += -D_USE_32BIT_TIME_T
 			BASIC_LDFLAGS += -Wl,--large-address-aware