[11u] RFR 8250928: JFR: Improve hash algorithm for stack traces

[Jaroslav Bachorík]

I would like to request a backport of this fix improving the hashing
function and reducing the chance of conflict in storing JFR stacktrace
data.

JIRA: https://bugs.openjdk.java.net/browse/JDK-8250928
Webrev: http://cr.openjdk.java.net/~jbachorik/8250928/11u/

The patch applies almost cleanly with only hunk offset.

Tier1 and JFR tests were executed to check for any regressions and all
expected tests passed.

Cheers,

-JB-