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

R=szuend@chromium.org

Bug: v8:12887
Change-Id: I8123d18ae852807557bf26b1308e0061dc1ac123
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3898937Reviewed-by: Simon Zünd <szuend@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83216}