Verify if Chromium's flag was enabled and toggle the flag
for V8, enabling support for PAC (Pointer Authentication Code)
and BTI (Branch Target Identification).

Bug: v8:10026, chromium:1145581, chromium:919548
Change-Id: I7c40674d2f9c8512639a7320b491006697420e28
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3072158
Commit-Queue: Martyn Capewell <martyn.capewell@arm.com>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76126}