Add CI testing for AArch64 Darwin

The Apple ARM silicon has been around for a while now and hopefully will
become Rust Tier 1 at some point. Add it to CI since it is distinct
enough from aarch64-linux and x86_86-darwin that there may be
differences.

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

@@ -9,6 +9,9 @@ jobs:
       fail-fast: false
       matrix:
         include:
+        - target: aarch64-apple-darwin
+          os: macos-14
+          rust: nightly
         - target: aarch64-unknown-linux-gnu
           os: ubuntu-latest
           rust: nightly
@@ -81,6 +84,8 @@ jobs:
           os: windows-latest
           rust: nightly-x86_64-gnu
     steps:
+    - name: Print runner information
+      run: uname -a
     - uses: actions/checkout@v4
       with:
         submodules: true