This CL changes the WASM implementation to access indirect function
tables through the WasmContext, whereas previously indirect function
tables and their sizes were constants that were inlined into compiled
into code, requiring code patching. This is a necessary step for sharing
code between instances and eventually, isolates.

R=clemensh@chromium.org,mstarzinger@chromium.org

Bug: v8:7424
Change-Id: Ida4138ed92729730dfbc0a81a84d8484b233d808
Reviewed-on: https://chromium-review.googlesource.com/895683
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51427}