Start runnning tests for aarch64-pc-windows-msvc

This target is currently build-only. Switch to the windows-11-arm
runner, which allows us to start running tests.

library/compiler-builtins/.github/workflows/main.yaml

@@ -51,8 +51,7 @@ jobs:
         - target: aarch64-unknown-linux-gnu
           os: ubuntu-24.04-arm
         - target: aarch64-pc-windows-msvc
-          os: windows-2025
+          os: windows-11-arm
-          build_only: 1
         - target: arm-unknown-linux-gnueabi
           os: ubuntu-24.04
         - target: arm-unknown-linux-gnueabihf