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

R=leszeks@chromium.org

Bug: v8:12887
Change-Id: I17a168a4810f13087be34a58358c684f1516da99
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3870489
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82980}