Shared array buffers are not tracked by the garbage collector, which
makes the fuzzer run out of memory pretty quickly. Since shared memory
is not needed any more for testing atomics, we can just make the memory
non-shared again.

This also improves the performance of the fuzzer (execs/s) by more than
2x locally.

R=ahaas@chromium.org

Bug: chromium:1281419
Change-Id: Ic7803617d6a14aaa698d9181327ec20b21d29faa
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3350764Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78418}