Updates IsARM64 to return true if __aarch64__ is defined, as the current
check only using _M_ARM64 only evaluates to true on Windows platforms.

Bug: chromium:1132295
Change-Id: I4853012e077217162ee3439c17fe35dcc6157e71
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2468787
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>