Change to v8/BUILD.gn regarding GN build of V8. Excluding platform-posix-time.h
and platform-posix-time.cc from the sources list on AIX os. The changes affect
AIX only.

Bug: chromium:616029
Change-Id: I5e5d46aa4c62f70544ff593294e459c607a03c67
Reviewed-on: https://chromium-review.googlesource.com/897124
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51183}