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

R=mlippautz@chromium.org

Bug: v8:12887
Change-Id: Id12f9d8270dd9fed651e1b3596c06e45aae9d6d1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3846151Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Auto-Submit: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82969}