RFR: 8205405: ZGC: Decouple JFR type registration

[Per Liden]

When JDK-8205053 is completed, it becomes possible to register 
subsystem-specific JFR types from outside of JFR. The code that 
registers ZGC's two types is currently embedded into JFR itself. This 
code can now be moved into ZGC itself.

Bug: https://bugs.openjdk.java.net/browse/JDK-8205405
Webrev: http://cr.openjdk.java.net/~pliden/8205405/webrev.0

Testing: Manually generated and verified a test recording

/Per