RFR: Move Shenandoah stubs generation into ShenandoahBarrierSetAssembler

[Roman Kennke]

This moves another bunch of Shenandoah code out of shared into
Shenandoah specific places. It removes a very significant amount of
upstream mess :-) BTW, ShenandoahBarrierSetAssembler is approaching 1000
LOC :-)

It builds on top of my previous patch to move runtime entries into
ShenandoahRuntime. This needs to go in first.

Testing: tier3_gc_shenandoah (ongoing, expected clean before push)

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

ok?

Roman