RFR: Make C1 write-barrier use a stub instead of dedicated instruction

[Roman Kennke]

We currently use a dedicated C1 LIR instruction to implement the
Shenandoah write barrier. This is not easily supported by the current
BarrierSetC1 interface. What works better is to use CodeStub (like all
other GC's barriers), so let's turn Shenandoah's write barrier (back) to
using CodeStubs. This removes a whole lot of upstream changes and
confines Shenandoah barrier stuff to gc/shenandoah.

http://cr.openjdk.java.net/~rkennke/c1-shenandoah-codestubs/webrev.01/

Testing: tier3_gc_shenandoah

Ok?