RFR (XS) 8222818: NMT summary could show the GC in use

[Eric Caspole]

Hi, could I have reviews and any opinions on this little change to show 
the GC name in the NMT output, as this helps us to more easily triage 
performance data.

This passed tier 1 and 2.
Thanks,
Eric


JBS:
https://bugs.openjdk.java.net/browse/JDK-8222818

webrev:
http://cr.openjdk.java.net/~ecaspole/JDK-8222818/02/webrev/