We were always using the instance we were currently building. If the
start function is an exported wasm function of another instance, use the
exporting instance instead.

R=titzer@chromium.org

Bug: chromium:843120
Change-Id: I141d272b947bef8e903be7208ddf6ce344e754c4
Reviewed-on: https://chromium-review.googlesource.com/1059620
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53190}