This extends the support for the "except_ref" type on global variables
to support mutable globals, as well as importing and exporting such
globals. Test coverage is also increased.

R=ahaas@chromium.org
TEST=mjsunit/wasm/exceptions-global
BUG=v8:8091

Change-Id: I816406e322ffb574a4f054947682491e7b40335f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1530802Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60327}