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

[Erik Österlund]

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

Thanks,
/Erik