RFR: JDK-8150367: Add back information about the number of GC workers

[Bengt Rutisson]

Hi everyone,

Could I have a couple of reviews for this change?

https://bugs.openjdk.java.net/browse/JDK-8150367

Before the changes to use the unified logging framework for the GC 
logging the number of GC worker threads was logged for each G1 
evacuation pause. It was printed as part of the "Parallel Time" line:

[Parallel Time: 5,5 ms, GC Workers: 23]

This information was lost in the conversion to the unified logging 
framework. This is relevant information and should be added back.

The proposed patch adds this back as a separate log line. That looks 
like this:

[1,647s][info][gc,task     ] GC(36) GC Workers: 23

The logging is done in AbstractWorkGang which is shared between G1 and 
CMS (ParNew). So, this information is now also logged for CMS.

Thanks,
Bengt