RFR: 8207756: jstat should show CGC STW phases of ZGC

[Yasumasa Suenaga]

Hi all,

Please review this change:

  JBS: https://bugs.openjdk.java.net/browse/JDK-8207756
  webrev: http://cr.openjdk.java.net/~ysuenaga/JDK-8207756/webrev.00/

jstat shows CGC STW phases as CGC and CGCT fields since JDK-8153333 .
However, ZGC does not adapt to it yet as following:

```
$ jstat -gc 1234 1 1
 S0C S1C S0U S1U EC EU OC OU MC MU CCSC CCSU YGC YGCT FGC FGCT CGC CGCT GCT
  - - - - - - 514048.0 12288.0 4352.0 3731.2 0.0 0.0 - - - - - - 0.000
```

I uploaded webrev which is for jdk/jdk repo. Please tell me if it
should be for zgc repo.
I'm not an author of ZGC. So I need a sponsor.
(I'm a Reviewer of jdk. If this change should be pushed to jdk/jdk,
please tell me. I will push it after reviewing.)


Thanks,

Yasumasa