Add support for compiling on Apple Silicon.

Defines new platform `darwin_arm64`.

RELNOTES: Add support to bazel/crosstool for building arm64 on macos aka darwin
PiperOrigin-RevId: 320064480

src/main/java/com/google/devtools/build/lib/rules/apple/ApplePlatform.java

@@ -56,7 +56,7 @@ public enum ApplePlatform implements ApplePlatformApi {
   // migrating it would be a breaking change more details:
   // https://github.com/bazelbuild/bazel/pull/7062
   private static final ImmutableSet<String> MACOS_TARGET_CPUS =
-      ImmutableSet.of("darwin_x86_64", "darwin");
+      ImmutableSet.of("darwin_x86_64", "darwin_arm64", "darwin");
 
   private static final ImmutableSet<String> BIT_32_TARGET_CPUS =
       ImmutableSet.of("ios_i386", "ios_armv7", "ios_armv7s", "watchos_i386", "watchos_armv7k");