This limits the call targets of {WasmCode} to only reference a limited
set of builtins via the {RelocInfo::CODE_TARGET} relocation mode. By now
most calls have been made independent of the Isolate.

R=clemensh@chromium.org
BUG=v8:7424

Change-Id: If4c643a7104e2ff0536ceb0f0de7fd30a923d854
Reviewed-on: https://chromium-review.googlesource.com/1092576Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53665}