This is a reland of bee5992a.
Fixes a TSan race report by replacing a FlagScope in tests with
direct assignment to the flag in question.

Original change's description:
> [wasm-gc] Initial Liftoff support
>
> This CL implements Liftoff support for struct.get/set,
> struct.new_with_rtt, rtt.canon, and ref.is_null, which
> is enough to make the first testcase pass.
>
> Bug: v8:7748
> Change-Id: Id09e9872d2126127192c852b3cb6d57ff9417582
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2584951
> Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
> Reviewed-by: Clemens Backes <clemensb@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#71744}

Bug: v8:7748
Change-Id: I17de6803c23a88209102385010dfdf9b88e25ace
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2593254
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71762}