This stages wasm code gc behind "--future" to get test coverage while
implementing this feature.

R=mstarzinger@chromium.org

Bug: v8:8217
Change-Id: Id620ee92518c4dd9cebc0fd47817bfc80e5cf3f5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1559741
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60722}