This makes the WebAssembly runtime stubs load the correct CEntry stub
from the instance object instead of embedding the address within the
instruction stream. It effectively makes those stubs independent of the
underlying Isolate.

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

Change-Id: I0e7f3ecf7642d3fb1ee7adf83a8f0e6cc4d38fdf
Reviewed-on: https://chromium-review.googlesource.com/1086997
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53575}