Slow asserts double the time spent in mksnapshot. While we want them
enabled on our bot builds, local builds should be fast and don't
necessarily need them.

This also adds v8_enable_fast_mksnapshot as default in gm.py.

Bug: v8:7891,v8:6688
Change-Id: I5ab1491f3396c4351de30dbbb80a81e2242c5db9
Reviewed-on: https://chromium-review.googlesource.com/1116552Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54074}