update build for Apple Silicon

You can use this to cross complie from an x86_64 mac if you have
at least XCode12 beta 2 installed, and you set target_cpu = "arm64"
in your gn args.

Change-Id: I3fcdcd162155ac0242c15260994de09177ff2f97
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/328659
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
3 files changed