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

R=tebbi@chromium.org

Bug: v8:12887
Change-Id: Ia636a23e8d9fceb9bc17d830b8395bac46984285
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3871202Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82982}