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

R=mliedtke@chromium.org

Bug: v8:12887
Change-Id: I36c66465e3b6c1b27c1825e50f17f4bc8557c426
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3898936Reviewed-by: Matthias Liedtke <mliedtke@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83217}