RFR: 8202547: Move G1 runtime calls used by generated code to G1BarrierSetRuntime

[Roman Kennke]

> Hi,
> 
> Generated code occasionally needs to call into the runtime for G1
> barriers. Some of these slow-path runtime calls are in SharedRuntime,
> some are in G1BarrierSet. It would be nice to have them collected in the
> same place. This patch move these slow-path C++ functions for G1 into a
> new class, g1BarrierSetRuntime.
> 
> Webrev:
> http://cr.openjdk.java.net/~eosterlund/8202547/webrev.00/
> 
> Bug:
> https://bugs.openjdk.java.net/browse/JDK-8202547
> 

This is a very nice cleanup. I was wondering the same just recently.
Patch looks fine. Thanks!

Roman