github: Stop specifying CMAKE_CXX_COMPILER_TARGET

The preinstalled CMake on the ARM64 runners now should be a native
ARM64 CMake, which allows autodetecting CMAKE_CXX_COMPILER_TARGET
correctly.

Author: mstorsjo

.github/workflows/test-libcxx.yml

@@ -85,7 +85,6 @@ jobs:
             -DLIBCXX_CXX_ABI=libcxxabi `
             -DCMAKE_C_COMPILER=${{matrix.target_arch}}-w64-mingw32-clang `
             -DCMAKE_CXX_COMPILER=${{matrix.target_arch}}-w64-mingw32-clang++ `
-            -DCMAKE_CXX_COMPILER_TARGET=${{matrix.target_arch}}-w64-windows-gnu `
             -DLIBCXXABI_ENABLE_SHARED=NO `
             -DLIBCXX_ENABLE_STATIC_ABI_LIBRARY=YES `
             -DLIBCXX_USE_COMPILER_RT=YES `