More code for the read barrier support.

Make it possible to disable the RB in Object::SizeOf() (and the
functions it calls transitively) which the collector will need to call
to get the size of an object when copying.

Add Object::AtomicSetReadBarrierPointer() for atomic write of a RB
pointer.

Bug: 12687968
Change-Id: Ibedd252860ac7ccd17e4e7d71b377a8892b48ff0