This sets us up for getting the wasm code generation off the GC heap.
We reference tables as global handles, which have a stable address. This
requires an extra instruction when attempting to make an indirect call,
per table (i.e. one for the signature table and one for the function
table).

Bug: 
Change-Id: I83743ba0f1dfdeba9aee5d27232f8823981288f8
Reviewed-on: https://chromium-review.googlesource.com/612322
Commit-Queue: Mircea Trofin <mtrofin@chromium.org>
Reviewed-by: Brad Nelson <bradnelson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47444}