This will allow us to target Big Sur for Mac ARM64 builds.

Bug: 1124119
Change-Id: I0aa17c744e13d45699e79eb17336ebcafb9ea719
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2777472Reviewed-by: Mark Mentovai <mark@chromium.org>
Reviewed-by: Jeff Yoon <jeffyoon@google.com>
Commit-Queue: Brian Ryner <bryner@google.com>