[gc] Add finalizer API, implemented only on sgen.

The new finalizer API allows an embedder to be notified during a collector
when an object is queued for finalization.

This is a VERY low level interface, as this is called with the world stopped, so the
code MUST be async safe. If you don't precisely know what that means, don't use it.
[boehm] Implement the finalization extension API.