This CL implements the global.get and global.set instruction for anyref
globals. This includes:

* Properly decode anyref globals.
* Add a FixedArray to WasmInstanceObject to store anyref globals.
* Initialize the FixedArray.
* Generate code for global.get and global set.

This CL does not allow to import globals yet.

R=clemensh@chromium.org

Bug: v8:7581
Change-Id: I62617409271d9b6f2253a191681189865aa1f459
Reviewed-on: https://chromium-review.googlesource.com/c/1380112Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58318}