If webassembly is disabled via a gn arg, we will not be able to enable
it via command-line switch. Hence make this flag read-only in that
configuration.

R=ecmziegler@chromium.org

Bug: v8:11238
Change-Id: Ib93a55f74d4f018477f110b8b52aa9b645e86553
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2710426Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Emanuel Ziegler <ecmziegler@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72905}