Global.set requires a write barrier. This write barrier was missing in
the original CL. The reland only adds the write barrier for ia32, and
bails out on the other platforms.

Original message:

With this CL we add support for all externref globals except for
imported mutable globals.

R=thibaudm@chromium.org, ulan@chromium.org

Bug: v8:7581
Change-Id: I86328a17200d1edc505f4c4357bdf795d95cf0c8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2404777Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69912}