RFR: 8189440: Event tracing macros for allocation and weak oops processing

[Erik Gahlin]

Hi,

Could I have a review of a change that adds two macros to be used with 
event-based JVM tracing.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8189440

Webrev:
http://cr.openjdk.java.net/~egahlin/8189440_0

Thanks
Erik