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/arch.go

@@ -308,7 +308,7 @@ var (
 	// LinuxMusl is the OS for the Linux kernel plus the musl runtime.
 	LinuxMusl = newOsType("linux_musl", Host, false, X86, X86_64)
 	// Darwin is the OS for MacOS/Darwin host machines.
-	Darwin = newOsType("darwin", Host, false, X86_64)
+	Darwin = newOsType("darwin", Host, false, Arm64, X86_64)
 	// LinuxBionic is the OS for the Linux kernel plus the Bionic libc runtime, but without the
 	// rest of Android.
 	LinuxBionic = newOsType("linux_bionic", Host, false, Arm64, X86_64)
@@ -696,6 +696,11 @@ func archMutator(bpctx blueprint.BottomUpMutatorContext) {
 	for i, m := range modules {
 		addTargetProperties(m, targets[i], multiTargets, i == 0)
 		m.base().setArchProperties(mctx)
+
+		// Install support doesn't understand Darwin+Arm64
+		if os == Darwin && targets[i].HostCross {
+			m.base().commonProperties.SkipInstall = true
+		}
 	}
 }