Extend gm.py to support long flags (starting with --), which are treated
as test runner flags, and passed unchanged. These flags must be as
single word, '--progress=verbose' instead of '--progress verbose', as gm
only does simple one-at-a-time args parsing.

Change-Id: Icfa161ff231715d0b7eb3ba259fca35a65c68964
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2250875
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68416}