[cmake] Pass PYTHON_EXECUTABLE to native builds (#163574)

Ensure that the nested native build uses the same python interpreter as
the main build, in case the python that CMake detects first is not the
python that the user has specified explicitly.

For example, if the person building LLVM wants to use a different python
interpreter to build (eg, testing the build with `python3.14` when
`python3` is a link to `python3.8`, or the default python doesn't have
development headers available) then they could add
`-DPYTHON_EXECUTABLE=python3.14` when invoking CMake. This should be
forwarded to the native CMake build to ensure that the same python is
used.

Original fix by Anuj Mittal <[email protected]>.

Author: rossburton

llvm/cmake/modules/CrossCompile.cmake

@@ -101,6 +101,7 @@ function(llvm_create_cross_target project_name target_name toolchain buildtype)
         -DLLVM_INCLUDE_BENCHMARKS=OFF
         -DLLVM_INCLUDE_TESTS=OFF
         -DLLVM_TABLEGEN_FLAGS="${LLVM_TABLEGEN_FLAGS}"
+        -DPYTHON_EXECUTABLE="${PYTHON_EXECUTABLE}"
         ${build_type_flags} ${linker_flag} ${external_clang_dir} ${libc_flags}
         ${ARGN}
     WORKING_DIRECTORY ${${project_name}_${target_name}_BUILD}