Remove a special case on mac hosts that was needed when is_clang wasn't
the default on Win, but is unnecessary now and gets in the way.

Bug: chromium:801780,chromium:495204
Change-Id: I8d6783c73e0c240704830a46304482e3ce94418b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1534634Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60415}