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

R=marja@chromium.org

Bug: v8:12887
Change-Id: Id315d33eee6b45e457766b0ba06c9d21c1e32807
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3899268
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83264}