This avoids embedding the {CEntryStub} into generated {WasmCode} and
instead loads it from the instance object. It is another step towards
making the generated code independent of the Isolate.

R=clemensh@chromium.org
BUG=v8:7472

Change-Id: Ic6ab7602a77fc11e6ec4a03e1bdba647d54df5e3
Reviewed-on: https://chromium-review.googlesource.com/1084841Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53491}