On old iPhones, jscvt is not availale. This CL diables jscvt on iOS in
general.

R=tebbi@chromium.org

Bug: v8:13004
Change-Id: Ib2651d7fa43892c06dc8c36e497a8c76344b5051
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3726297Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81462}