Add OSX arm64 build target.

New "macos_arch" to specify the desired arch, and "macos_sdk_path" to
override the default SDK path.

.github/workflows/ci.yml

@@ -145,6 +145,33 @@ jobs:
         run: |
           ./Godot.app/Contents/MacOS/Godot --path test -s script.gd
 
+  macos-arm64:
+    name: Build (macOS, Clang, cross-compile arm64)
+    runs-on: macos-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2.3.4
+        with:
+          submodules: recursive
+
+      - name: Set up Python (for SCons)
+        uses: actions/setup-python@v2
+        with:
+          python-version: '3.9.1'
+
+      - name: Install dependencies
+        run: |
+          python -m pip install scons
+
+      - name: Build godot-cpp
+        run: |
+          # The default SDK in github the actions environemnt seems to have problems compiling for arm64.
+          # Use the latest 11.x SDK.
+          SDK_BASE=/Library/Developer/CommandLineTools/SDKs
+          SDK_VER=$(ls $SDK_BASE | grep "MacOSX11." | sort -r | head -n1)
+          echo $SDK_BASE/$SDK_VER/
+          scons target=release generate_bindings=yes macos_arch=arm64 macos_deployment_target=10.15 macos_sdk_path="$SDK_BASE/$SDK_VER/" -j $(sysctl -n hw.logicalcpu)
+
   static-checks:
     name: Static Checks (clang-format)
     runs-on: ubuntu-20.04