[macOS] Add support for universal builds.

Since the OpenSSL build system does not support macOS universal
binaries, we first need to build the two libraries separately, then we
join them together using lipo.

.github/workflows/build_release.yml

@@ -84,17 +84,11 @@ jobs:
 
           # macOS
           - platform: macos
-            arch: 'x86_64'
-            gdnative_flags: 'macos_arch=x86_64 bits=64'
+            arch: 'universal'
+            gdnative_flags: 'macos_arch=universal bits=64'
             sconsflags: ''
             os: 'macos-11'
-            cache-name: macos-x86_64
-          - platform: macos
-            gdnative_flags: 'macos_arch=arm64 bits=64'
-            arch: 'arm64'
-            sconsflags: ''
-            os: 'macos-11'
-            cache-name: macos-arm64
+            cache-name: macos-universal
 
           # Windows
           - platform: windows