RFR: Consolidate assembly write barriers

[Roman Kennke]

We have two versions of write-barrier-implementations in assembly code:
one for interpreter and one for C1 (and C2, but this is going away with
previous RFR). The reason why we had this was that the stub that is used
by the 2nd version could previously not be generated before the
interpreter. However, with recent enhancements by the modularization of
barriers efforts, this is fixed, and we can now use the same
write-barrier-assembly everywhere.

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

this moves the WB code from MacroAssembler into
ShenandoahBarrierSetAssembler, let's all WB impls go through the stubs,
let's all RT calls go through the JRT_LEAF entry.

Testing: tier3_gc_shenandoah

Ok (pending review of "Remove C2 write-barrier from .ad files" which it
depends on)?

Thanks,
Roman