Move all Shenandoah runtime entry points to ShenandoahRuntime

[Roman Kennke]

As $SUBJECT says. In particular, it moves the shenandoah_clone_barrier
out of SharedRuntime, thus reducing our upstream mess.

http://cr.openjdk.java.net/~rkennke/shenandoahrt/webrev.00/

Testing: tier3_gc_shenandoah (ongoing, expected clean before push)

Ok?