RFR: 8230431: Move G1 trace code from gcTrace* to G1 directory

[Stefan Johansson]

Hi all,

Please review this cleanup to prepare for fixing JDK-8209802.

JBS: https://bugs.openjdk.java.net/browse/JDK-8230431
Webrev: http://cr.openjdk.java.net/~sjohanss/8230431/00/

Summary:
Currently a lot of JFR related gc code is located in share/gcTrace.* and share/gcTraceSend.cpp. To fix JDK-8209802, we need to have a way to initialize and register a few JFR constants specific to G1 and to prepare for this I suggest that we move the G1 parts from the above files into g1/g1Trace.* to create a place where this can be done. This is something that probably should have been done regardless of this bug, but now is a good to get it done. 

Testing:
Mach5 tier 1-3

Cheers,
Stefan