[8u] RFR 8250928: JFR: Improve hash algorithm for stack traces

[Jaroslav Bachorík]

Please, review this backport of a hash-collision reducing fix in JFR
stacktrace values from the main line.

JIRA: https://bugs.openjdk.java.net/browse/JDK-8250928
Webrev: http://cr.openjdk.java.net/~jbachorik/8250928/8u/webrev/

The original patch had to be adjusted for the fact that the hash code
is calculated in jfrStackTraceRepository.cpp instead of
jfrStackTrace.cpp which does not exist in 8u.

The gist stays the same - the hash code computation is updated to the
algorithm used in the patch.

Tests from tier1 and JFR were run.

Cheers,

-JB-