ci: Add a build configuration, testing building for iOS

Author: mstorsjo

.github/workflows/build.yaml

@@ -114,6 +114,15 @@ jobs:
       - run: c++ --version
       - run: CFLAGS=-Werror make -j$(sysctl -n hw.ncpu) ARCH=arm64
 
+  ios-cross-arm64:
+    runs-on: macos-latest
+    steps:
+      - uses: actions/checkout@v3
+      - run: make gmp-bootstrap
+      - run: make gtest-bootstrap
+      - run: c++ --version
+      - run: CFLAGS=-Werror make -j$(sysctl -n hw.ncpu) OS=ios ARCH=arm64
+
   fuzzing:
     runs-on: ubuntu-latest
     steps: