on ppc64/s390x, gcc 6.3 complains
"error: unknown option after '#pragma GCC diagnostic' kind".

This is to avoid using the option on these platforms.

R=jkummerow@chromium.org

Bug: v8:8100

Change-Id: I61e5b70038c41cb62889a36694ffcc36fa6deee9
Reviewed-on: https://chromium-review.googlesource.com/1246803Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#56273}