Add Darwin+Arm64 toolchain support

This just sets up the toolchain and allows Darwin+Arm64 to be specified
as a HostCross target. These variants will not be exported to Make, or
be installed on a Soong-only build. A future CL will add support for
universal binaries using these variants.

This config is a bit stranger than the regular 64/32 multilib, as it's
two primary 64-bit configs. And on a Darwin/X86 machine, the Arm64
versions are HostCross (doesn't work on the current machines), while a
Darwin/Arm64 machine, either version works (if Rosetta is installed).

Bug: 203607969
Change-Id: Iacaed77d267773672da027cd74917e33fb1c1e94

android/androidmk.go

@@ -921,6 +921,11 @@ func shouldSkipAndroidMkProcessing(module *ModuleBase) bool {
 		return true
 	}
 
+	// Only expose the primary Darwin target, as Make does not understand Darwin+Arm64
+	if module.Os() == Darwin && module.Target().HostCross {
+		return true
+	}
+
 	return !module.Enabled() ||
 		module.commonProperties.HideFromMake ||
 		// Make does not understand LinuxBionic