[8u] RFR 8252754: Hash code calculation of JfrStackTrace is inconsistent

[Jaroslav Bachorík]

Please review this simple backport.

The patch had to be adjusted for the fact that the stack trace hash
code is calculated in jfrStackTraceRepository.cpp instead of the
original jfrStackTrace.cpp. The hash code calculation logic remains
the same, though.

JIRA: https://bugs.openjdk.java.net/browse/JDK-8252754
Webrev: http://cr.openjdk.java.net/~jbachorik/8252754/8u/webrev/

Cheers,

-JB-