github: Run test-llvm on aarch64 too

As the preinstalled CMake on the windows-11-arm runners currently
is an x86_64 build, we need to explicitly specify
CMAKE_SYSTEM_PROCESSOR, and CMAKE_SYSTEM_NAME to make CMake
consider it a cross compilation (otherwise CMAKE_SYSTEM_PROCESSOR
gets overridden by CMAKE_HOST_SYSTEM_PROCESSOR, which gets influenced
by the x86_64 as CMake is running emulated).

We also need to set LLVM_HOST_TRIPLE, as the autodetection of
it relies on CMAKE_HOST_SYSTEM_PROCESSOR. The host triple needs
to be in normalized form with "-windows-gnu", otherwise a couple
dozens of tests fail.

We also include the AArch64 target, to make sure the built compiler
can generate code for the host.

Since 07bc54bf4554398b199f4dc849e5193b98422f23 in llvm-project
(May 5th), these tests should pass on aarch64 too.

.github/workflows/test-llvm.yml

@@ -41,7 +41,13 @@ jobs:
 
   test-llvm:
     needs: [prepare]
-    runs-on: windows-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        arch:
+          - x86_64
+          - aarch64
+    runs-on: ${{startsWith(matrix.arch, 'a') && 'windows-11-arm' || 'windows-latest'}}
     steps:
       - name: Install dependencies
         run: |
@@ -54,7 +60,7 @@ jobs:
           commit: ${{inputs.commit}}
           branch: ${{inputs.branch}}
           event: ${{inputs.pipeline_type}}
-          name: windows-ucrt-x86_64-toolchain
+          name: windows-ucrt-${{matrix.arch}}-toolchain
       - name: Unpack toolchain
         run: |
           Expand-Archive llvm-mingw-*.zip -DestinationPath .
@@ -80,7 +86,10 @@ jobs:
           cmake ../llvm `
             -G Ninja `
             -DCMAKE_BUILD_TYPE=Release `
-            -DLLVM_TARGETS_TO_BUILD=X86 `
+            -DLLVM_HOST_TRIPLE=${{matrix.arch}}-w64-windows-gnu `
+            -DCMAKE_SYSTEM_PROCESSOR=${{matrix.arch}} `
+            -DCMAKE_SYSTEM_NAME=Windows `
+            -DLLVM_TARGETS_TO_BUILD="AArch64;X86" `
             -DLLVM_ENABLE_PROJECTS="clang;lld;clang-tools-extra" `
             -DLLVM_WINDOWS_PREFER_FORWARD_SLASH=OFF `
             -DCMAKE_C_COMPILER=clang `