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}