RFR: 8u backport of JDK-8237499 JFR: Include stack trace in the ThreadStart event

[Yang Yi]

Hi,

May I have a request backport of JDK-8237499?

JBS: https://bugs.openjdk.java.net/browse/JDK-8237499
Webrev: http://cr.openjdk.java.net/~ddong/yiyang/8237499/
testing: jdk/test/jdk/jfr/

It's useful for users to know where their threads start. This patch doesn't apply 
cleanly but is fairly trivial. The latest jdk8u-dev has no `Thread::current_or_null` 
function, just adding this function solves the problem.

This patch will cause another problem, e.g., the minimized VM fails to build, this 
problem has been solved by JDK-8239886, so a follow-up backport of 
JDK-8239886(one-line change) will be sent later.

Cheers,
Yang Yi