PPC/s390: enable gn build on s390x and ppc64le

Bug: 
Change-Id: Iac1ad60e8aa4f5ac3fac9069a2f958cb1449d03c
Reviewed-on: https://chromium-review.googlesource.com/747534
Commit-Queue: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Leon Scroggins <scroggo@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Reviewed-by: Chris Palmer <palmer@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#513500}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 1dc34c0d9553339cb54970c7aadd05aa5c81dd57