Also enable test runner to differentiate between clang and gcc.

Bug: v8:8919
Change-Id: Icdcae0aba3644a1b1b9ddc6c037eabde27d717f7
Reviewed-on: https://chromium-review.googlesource.com/c/1491634Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Sergiy Belozorov <sergiyb@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59906}