When processing imports of an instance, we were storing pointers to
exported (and re-imported) wasm functions in the code table of the
importing module. This is dangerous since imports are instance specific.
Avoid ever storing call targets for imports in the NativeModule.
Instead, read the call targets from the imports table of the instance.

R=mstarzinger@chromium.org

Bug: chromium:843563

Change-Id: Id9f43a6c127025a5feaa81b2be75c001bc0bea81
Reviewed-on: https://chromium-review.googlesource.com/1065774
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53256}