This adds basic support and tests for having global variables of type
"except_ref" that are default initialized to "null". The functionality
is part of the exception handling proposal and solely enabled by the
corresponding feature flag.

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

Change-Id: I581bc942fbe6688a5c58790a842fb024de22d924
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1528439
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60317}