Draft: rts: Make LOOKS_LIKE_INFO_PTR a bit more strict

In particular, we now verify that the info table doesn't reside in the dynamic heap.

Test Plan: Validate

Reviewers: simonmar, osa1

Reviewed By: osa1

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4759