Add support for emitting GC-ness of the second return register for 16 byte

Add support for emitting GC-ness of the second return register for 16 byte
structs.

This changeset adds support for emitting the GC-ness of the second return
register (RDX for System V Amd64) for multi-register return structs.
It closes a hole in the GC info and resolves multiple GC stress failures.

Fixes #2757.