Latest C2 modularization work

[Erik Österlund]

Greetings,

Since last time, my C2 modularization patches were upstreamed in 
mainline, and conditional ZGC compilation was added in the ZGC repo. 
Here is my latest attempt of adapting the ZGC code to the new C2 
interface in an attempt to make upstream as tidy as possible.

Step 1: Cherry pick BarrierSetC2 from upstream, with my-conflict into 
current ZGC repo. This is an intermediate step that does not build.
http://cr.openjdk.java.net/~eosterlund/zgc_c2/webrev.01.00/

Step 2: Move everything into ZBarrierSetC2. This should build and run.
http://cr.openjdk.java.net/~eosterlund/zgc_c2/webrev.01.01/

Step 3: Move the x86 slow-path stub generation to ZBarrierSetAssembler. 
This should also build and run.
http://cr.openjdk.java.net/~eosterlund/zgc_c2/webrev.01.02/

Steps 1-3 combined:
http://cr.openjdk.java.net/~eosterlund/zgc_c2/webrev.01.00_02/

This could do with some more testing cycles still. Then I think it 
should be folded in for the next rebase.

Thanks,
/Erik