[Flags] Add flag for wasm code gc

This adds a flag for garbage collection of WebAssembly code. It will be
used in a finch experiment, and expire once code GC is shipped to
stable.

R=titzer@chromium.org

Bug: v8:8217
Change-Id: I5ab3e230c10fbd0146019547301d24c0e5a80396
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1578540
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653940}
8 files changed