baby steps for aarch64 support

So far this is just as easy as I had hoped.

Change-Id: I5f69a900b32d9bf70156b55e334233d7376b820f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/223340
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
3 files changed