We need to change WasmExportedFunction to call imported functions via
the import table, so there will be no embedded call target.
This also removes the necessity to generate an unreachable call after
the runtime call for js-incompatible signatures.

R=titzer@chromium.org

Bug: chromium:843563,v8:6668
Change-Id: I82cb31930f6b61ad59fde63a8c5ae631da3d1a14
Reviewed-on: https://chromium-review.googlesource.com/1063771
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53239}