Building arm64 binaries on arm64 hosts works as long as you set
the correct options in args.gn. This patch teaches gm.py to do
that.
Building 32-bit arm binaries on arm64 hosts requires an extra
definition in snapshot_toolchain.gni (as well as some system
setup to support running 32-bit binaries).

Change-Id: I66c1f8f51932e2f5425033ef09181c31ea5d633e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2743889
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73323}