RFR: 8237499: JFR event for the location where a new thread starts

[Denghui Dong]

Hi,

Could I have a review of a change that introduces an event for the location where a new thread starts.

Summary:
There is already an event for thread start(EventThreadStart), but this event is committed in the new thread,
so users can't know the location where the thread starts, hence, it's necessary to introduce another event, 
it would be useful if the application exists thread leak problem.

Bug: https://bugs.openjdk.java.net/browse/JDK-8237499
Webrev: http://cr.openjdk.java.net/~ddong/8237499/

Thanks
Denghui Dong