The SerializeGeneric method assumes that the object was not serialized
before. Hence, we should not call it repeatedly for the same builtin.
This CL now exposes SerializeBuiltin, and calls that directly.
We also serialize the Illegal builtin for wasm interpreter entries,
which are never reused across instantiations anyway.

R=ahaas@chromium.org, yangguo@chromium.org
BUG=v8:5822

Change-Id: Id74b86fe29171908ed35ddbc06c93f0d241e4917
Reviewed-on: https://chromium-review.googlesource.com/458380Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44155}