Implement finalization.

Also make System.gc no longer a no-op. This replaces some of the
MemberOffsets exposed by Heap with intention-revealing functions
that we'll want to share between all the actual GC implementations,
but there's still more to be done.

Change-Id: I57ba26c0416fbbfe20142b7b6e27108684add7f9
10 files changed
tree: f70ad27d5fc007b70765a76644519234466dee61
  2. build/
  3. oat_process/
  4. oat_runtime/
  5. src/
  6. test/
  7. tools/