x86-64 Baker's read barrier fast path implementation.

Introduce an x86-64 fast path implementation in Optimizing
for Baker's read barriers (for both heap reference loads and
GC root loads).  The marking phase of the read barrier is
performed by a slow path, invoking the runtime entry point
artReadBarrierMark.

Other read barrier algorithms continue to use the original
slow path based implementation, which has been renamed as
GenerateReadBarrierSlow/GenerateReadBarrierForRootSlow.

Bug: 12687968
Change-Id: I9329293ddca7f9bcb512132bde6675aa202b98b2