[zircon] Add VM exec-setters API

Initial stub for supporting non-executable memory following Mark
Seaborn's exec-setter proposal. This API is still very permissive and
insecure at the moment to allow userspace to start incrementally
migrating to using them.

SEC-42

Test: Added vmo_replace_as_executable to core-tests.
Change-Id: I7ad2f6451b230853d919121f1ef7255be2ca2941