MSVC only defines _M_ARM64 to identify Windows ARM64, so detect this macro is
necessary to building WebRTC with MSVC.
Bug: chromium:893460
Change-Id: Idd49dfb6dcf367d5efdec705b84db55087de37c1
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/154281
Commit-Queue: Tommi <tommi@webrtc.org>
Reviewed-by: Tommi <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29280}