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

R=dinfuehr@chromium.org

Bug: v8:12887
Change-Id: Icc9e1d2db58999b676477924284f78043cf5533c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3899124Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83263}