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

SConstruct

@@ -143,6 +143,17 @@ opts.Add(
     'macOS deployment target',
     'default'
 )
+opts.Add(
+    'macos_sdk_path',
+    'macOS SDK path',
+    ''
+)
+opts.Add(EnumVariable(
+    'macos_arch',
+    'Target macOS architecture',
+    'x86_64',
+    ['x86_64', 'arm64']
+))
 opts.Add(EnumVariable(
     'ios_arch',
     'Target iOS architecture',
@@ -217,14 +228,19 @@ elif env['platform'] == 'osx':
             'Only 64-bit builds are supported for the macOS target.'
         )
 
-    env.Append(CCFLAGS=['-std=c++14', '-arch', 'x86_64'])
+    env.Append(CCFLAGS=['-std=c++14', '-arch', env['macos_arch']])
 
     if env['macos_deployment_target'] != 'default':
         env.Append(CCFLAGS=['-mmacosx-version-min=' + env['macos_deployment_target']])
+        env.Append(LINKFLAGS=['-mmacosx-version-min=' + env['macos_deployment_target']])
+
+    if env['macos_sdk_path']:
+        env.Append(CCFLAGS=['-isysroot', env['macos_sdk_path']])
+        env.Append(LINKFLAGS=['-isysroot', env['macos_sdk_path']])
 
     env.Append(LINKFLAGS=[
         '-arch',
-        'x86_64',
+        env['macos_arch'],
         '-framework',
         'Cocoa',
         '-Wl,-undefined,dynamic_lookup',