This CL adds support for anyref in WebAssembly.Global objects. Note
that the specification is not complete yet in this area.

I did the following changes:
- I renamed the `array_buffer` field of WasmGlobalObject to
  `untagged_buffer`
- I added an additional field of type FixedArray, `tagged_buffer`.
  - In the constructor of WasmGlobalObject I allocate either the former
    or the latter, but not both.
- In the WebAssembly.Global constructor I added special handling for
  the case where no initial value is provided. In that case I set the
  inital value to `null` and not to `undefined`.

R=titzer@chromium.org

Bug: v8:7581
Change-Id: I7e4855d7e6c04a9bcdc7ebd450caca5819d060e2
Reviewed-on: https://chromium-review.googlesource.com/c/1398226
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58625}