github: Run test-libcxx on ARM too

This requires setting CMAKE_CXX_COMPILER_TARGET manually; the LLVM
build system infers the wrong architecture when CMake is an
x86_64 binary.

(The wrong inferred architecture doesn't affect building libcxx,
but the wrong architecture gets passed as --target= parameter
when running the tests.)

Since 9c67ae861ccb0312fc680414a8441892d169d14a in llvm-project
(May 9th), these tests should pass on aarch64 too.

Co-authored-by: Jeremy Drake <github@jdrake.com>

.github/workflows/test-libcxx.yml

@@ -41,13 +41,15 @@ jobs:
 
   test-libcxx:
     needs: [prepare]
-    runs-on: windows-latest
     strategy:
       fail-fast: false
       matrix:
         include:
-          - { arch: x86_64, prefix: i686-w64-mingw32- }
-          - { arch: x86_64, prefix: }
+          - { arch: x86_64,  target_arch: i686    }
+          - { arch: x86_64,  target_arch: x86_64  }
+          - { arch: aarch64, target_arch: armv7   }
+          - { arch: aarch64, target_arch: aarch64 }
+    runs-on: ${{startsWith(matrix.arch, 'a') && 'windows-11-arm' || 'windows-latest'}}
     steps:
       - name: Install dependencies
         run: |
@@ -87,8 +89,9 @@ jobs:
             -DLIBCXX_ENABLE_WERROR=YES `
             -DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi;libunwind" `
             -DLIBCXX_CXX_ABI=libcxxabi `
-            -DCMAKE_C_COMPILER=${{matrix.prefix}}clang `
-            -DCMAKE_CXX_COMPILER=${{matrix.prefix}}clang++ `
+            -DCMAKE_C_COMPILER=${{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 `