platform.machine() is 'arm64' on arm macs, and the `.startswith('arm')`
branch converted that to 'arm' before this CL.

Bug: 1103236,1190880
Change-Id: Idd75a724f059ecd2dd873737e4998fe9bc937e04
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2779414
Commit-Queue: Nico Weber <thakis@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@google.com>
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>