mac: enough to make `target_cpu = "arm64"` make it through `gn gen`

Doesn't build at all, but hey, it's a start.

Bug: 1098899
Change-Id: I63500b3650b315c47219614cc9ae9a3ddbcf42c5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2264499


Commit-Queue: Mark Mentovai <mark@chromium.org>
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#782116}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 0347dc10a71fbb47457f639f7f908bdeaafbebc8