This makes the fact that export wrapper code is shared across instances
explicit by hanging the {export_wrappers} array off the module object
instead of the instance-specific {WasmCompiledModule} object.

R=titzer@chromium.org

Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: Ic5c73bcc17f759e520c105317361e5654628b99e
Reviewed-on: https://chromium-review.googlesource.com/1051987
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53131}