Some embedders (e.g. ChromeOS) override the toolchain and do not
provide such macros.

Bug: chromium:1056170
Change-Id: I3012af6dd7f59580d00e58608a228df562706414
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2132798Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66961}