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
diff --git a/bazel/configurability.go b/bazel/configurability.go
index e9641e7..f05c8e5 100644
--- a/bazel/configurability.go
+++ b/bazel/configurability.go
@@ -39,6 +39,7 @@
 	osArchAndroidArm64      = "android_arm64"
 	osArchAndroidX86        = "android_x86"
 	osArchAndroidX86_64     = "android_x86_64"
+	osArchDarwinArm64       = "darwin_arm64"
 	osArchDarwinX86_64      = "darwin_x86_64"
 	osArchLinuxX86          = "linux_glibc_x86"
 	osArchLinuxX86_64       = "linux_glibc_x86_64"
@@ -96,6 +97,7 @@
 		osArchAndroidArm64:         "//build/bazel/platforms/os_arch:android_arm64",
 		osArchAndroidX86:           "//build/bazel/platforms/os_arch:android_x86",
 		osArchAndroidX86_64:        "//build/bazel/platforms/os_arch:android_x86_64",
+		osArchDarwinArm64:          "//build/bazel/platforms/os_arch:darwin_arm64",
 		osArchDarwinX86_64:         "//build/bazel/platforms/os_arch:darwin_x86_64",
 		osArchLinuxX86:             "//build/bazel/platforms/os_arch:linux_glibc_x86",
 		osArchLinuxX86_64:          "//build/bazel/platforms/os_arch:linux_glibc_x86_64",