According to the spec, exported wasm functions should not have a
[[Construct]] method, hence they don't have a prototype.

R=bmeurer@chromium.org
CC=​titzer@chromium.org

Bug: v8:7503
Change-Id: I9e142d65a80c0ef6dbd743421771f194c2d50614
Reviewed-on: https://chromium-review.googlesource.com/939782Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51622}