Use functions instead of macros for platform detection so compilation can be
checked from any platform.  Also makes the workaround detection more readable.

BUG=angleproject:3026

Change-Id: I4153f0a2a6a6d5860c7b37f7cc67561895165ed1
Reviewed-on: https://chromium-review.googlesource.com/c/1378685
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>