This fixes initialization of reference type global variables (i.e.
anyref and except_ref) based on an index of another global. It extends
the existing support to exception types, fixes the logic, and also fixes
a missing write barrier.

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

Change-Id: Ia91f1ea03be24fadf3023a5acdd073badb8dcd93
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1539581
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60525}