During builtins generation, parts of the builtins table may be filled
with placeholder code objects.

This CL ensures that such placeholders are replaced by the real
builtin object during finalization of the builtins constants table.

Bug: v8:6666
Change-Id: I3a2635b29b37690fd7e950b9f38d500704671afb
Reviewed-on: https://chromium-review.googlesource.com/934241Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51498}