RFR: JDK-8066441 - Add PLAB trace event

[Staffan Friberg]

Hi,

As noted in the original thread [1] about this event we split up the 
commit in 4 different steps. This is the first step that only adds the 
event and methods to send them, but the usage will be added separately 
for the 3 GCs using PLABs.

Bug (sub-task): https://bugs.openjdk.java.net/browse/JDK-8066441
Webrev: http://cr.openjdk.java.net/~sfriberg/JDK-8066441/webrev.00

Thanks,
Staffan

[1] - (RFR: JDK-8055845 - Add trace event for promoted objects) - 
http://mail.openjdk.java.net/pipermail/hotspot-gc-dev/2014-December/011477.html