RFR: 8003216: Add JFR event indicating explicit System.gc() cal

[Erik Gahlin]

Hi,

Could I have a review of a JFR event that is emitted when System.gc() is 
called.

Purpose is to collect the stack trace. It is not sufficient with the 
cause field that the GarbageCollection event has today.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8003216

Webrev:
http://cr.openjdk.java.net/~egahlin/8003216/

Testing:
tier1+tier2+jdk/jdk/jfr

Thanks
Erik