github: Run test-compiler-rt on ARM too

Since 3e56f5f41843204b6e612362542b77110f8f8188 in llvm-project,
this should pass for all 4 architectures. (This doesn't pass
in the pinned release version right now.)

.github/workflows/build.yml

@@ -500,19 +500,21 @@ jobs:
           ninja
           ninja check-openmp
 
-  # Run the compiler-rt tests with the cross-built i686/x86_64 toolchains from
-  # above. This also forces testing the bundled python executables.
+  # Run the compiler-rt tests with the cross-built toolchains from above. This
+  # also forces testing the bundled python executables.
   test-compiler-rt:
     # Only running these tests in scheduled builds.
     if: (github.event_name == 'schedule') && (github.repository == 'mstorsjo/llvm-mingw')
     needs: [linux-cross-windows, prepare]
-    runs-on: windows-latest
     strategy:
       fail-fast: false
       matrix:
         arch:
           - x86_64
           - i686
+          - armv7
+          - aarch64
+    runs-on: ${{startsWith(matrix.arch, 'a') && 'windows-11-arm' || 'windows-latest'}}
     steps:
       - name: Install dependencies
         run: |