RFR(xxs): 8157271: Avoid extra copy of NativeCallStack

[Max Ockner]

Hello,

Please review this one line enhancement. We formerly passed a copy of a 
NativeCallStack into MallocSiteHashtableEntry. We now pass a reference 
instead.

bug: https://bugs.openjdk.java.net/browse/JDK-8157271
webrev: http://cr.openjdk.java.net/~mockner/8157271.01/

tested with runtime/NMT jtreg tests.

Thanks,
Max