Rename the flag --liftoff-extern-ref to
--experimental-liftoff-extern-ref to keep the fuzzer from using it.
The implementation is not complete yet, and the next steps may take a
bit.

R=clemensb@chromium.org

Bug: chromium:1137601
Change-Id: I74f1ed8faba44e42f63790d87f4a538dd59ac852
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2465838Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70476}