This patch adds two improvements to gm.py:
(1) Fix "mksnapshot failure" detection to match the error message
    that is generated when embedded builtins are enabled.
(2) Run "gn gen" whenever build.ninja is missing. This can happen
    when out/<config>/args.gn exists already when gm.py is run for
    the first time.

Bug: v8:6666, v8:8335
Change-Id: I71836b832754fa21b6443d57a6c3c49718a9a8d1
Reviewed-on: https://chromium-review.googlesource.com/c/1294174Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56918}