[8u] RFR 8229420: [Redo] jstat reports incorrect values for OU for CMS GC

[Zhengyu Gu]

Please review this 8u backport, as this CR is on Oracle's 8u backport.

The original patch does not apply cleanly, because there are a few 
general GC refactorings, as well as CMS refactorings.


Original Bug: https://bugs.openjdk.java.net/browse/JDK-8229420
Original code review thread: 
https://mail.openjdk.java.net/pipermail/serviceability-dev/2019-August/028935.html


8u backport: http://cr.openjdk.java.net/~zgu/JDK-8229420_8u/webrev.00/

Test:
  gc jtreg tests on Linux x86_64


Thanks,

-Zhengyu