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

R=cbruni@chromium.org

Bug: v8:12887
Change-Id: I7e828480e9cc919609dac69df89315c6fdc82dff
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3899296Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83228}