ZGC C2 changes

[Erik Österlund]

Greetings,

I have done some preliminary work to adapt ZGC to the new BarrierSetC2 
changes on hotspot-dev.

Here is a patch cherry picking that into ZGC land:
http://cr.openjdk.java.net/~eosterlund/zgc_c2/webrev.00/

I basically chose my BarrierSetC2 conflict in the merge. This 
intermediate step does not compile.

After that, the following patch moves >1500 LOC of ZGC-specific compiler 
poop into ZBarrierSetC2:

http://cr.openjdk.java.net/~eosterlund/zgc_c2/webrev.01/

Here is a full webrev with the result of BarrierSetC2 + ZBarrierSetC2 
dumped into our repo.

I still need to run a bunch of more tests on this to verify I have not 
messed things up, but thought I'd throw this out there for faster turn 
around times so that you can have a look at it. The BarrierSetC2 patch 
has made its way through hs-tier1 to hs-tier5. There are still traces 
left of ZGC in the C2 code, but the large bulk of GC code has been moved 
out of the way. But we need to move the stub generation to 
ZBarrierSetAssembler::barrier_stubs_init() and stuff like that.

Thanks,
/Erik