CI: macOS M1 architecture

Add `macos-14` target for building on Apple Silicon (M1)
macOS.

See the following url for details:
https://github.blog/changelog/2024-01-30-github-actions-introducing-the-new-m1-macos-runner-available-to-open-source/

.github/workflows/stable.yml

@@ -87,7 +87,12 @@ jobs:
           make -C quiche/examples
 
   quiche_macos:
-    runs-on: macos-latest
+    strategy:
+      matrix:
+        target:
+          - "macos-latest" # Intel (x86_64)
+          - "macos-14"     # Apple Silicon (M1)
+    runs-on: ${{ matrix.target }}
     # Only run on "pull_request" event for external PRs. This is to avoid
     # duplicate builds for PRs created from internal branches.
     if: github.event_name == 'push' || github.event.pull_request.head.repo.full_name != github.repository