Do allocation fence before pushing on allocation stack

Heap::VisitObjects relies on having valid classes for objects in
the allocation stack. If the writes reorder, the thread calling
VisitObjects could see the free list pointer instead of the class
of the object. I believe this is causing crashes in VisitObjects.

Bug: 28790624

Test: Volantis booted

Change-Id: I0f2d4097de1ef3f5caf670ecc977d4d6837872ca