RFR JDK-8066443: Add G1 support for promotion event

[Staffan Friberg]

Hi,

This webrev adds support for the G1 collector and the promotion event 
that collects statistics about the promoted object types.

Once reviewed I would need a sponsor to push the patch.

Bug: https://bugs.openjdk.java.net/browse/JDK-8066443
Webrev: http://cr.openjdk.java.net/~sfriberg/JDK-8066443/webrev.01/

//Staffan