Avoid the deprecated FLAG_* syntax, access flag values via the
{v8_flags} struct instead.

R=jkummerow@chromium.org

Bug: v8:12887
Change-Id: Ia17d668b3ddcbcb7a35388231aa5d80e8e5b419b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3899122
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83270}