Fix for 8013633, fix platform ifdefs in unsafe.cpp

[bill.pittore at oracle.com]

Rather than add yet another platform to the ifdefs in unsafe.cpp we 
thought it would be cleaner to reference
#ifdef SUPPORTS_NATIVE_CX8

which is set in platform specific globalsDefinitions_<arch>.hpp file

Webrev is here:
http://cr.openjdk.java.net/~bpittore/8013633/webrev.00/

thanks,
bill