RFR 8014911 - Use SUPPORTS_NATIVE_CX8 to help C++ compiler elide code

[BILL PITTORE]

Would like a review for this simple fix that leverages the macro 
'SUPPORTS_NATIVE_CX8' that was added in hotspot revision:
changeset:   4611:6b388e7d4905
parent:      4547:e01e02a9fcb6
user:        bpittore
date:        Tue May 07 10:19:33 2013 -0400
summary:     8013633: Cleanup platform ifdefs in unsafe.cpp

SUPPORTS_NATIVE_CX8 is defined in globalDefinitions_<arch>.hpp based on 
whether the platform supports 64bit cmpxchg natively. This change uses 
the define to force the supports_cx8() function to return hard-coded 
'true' value. GCC and other compilers will then optimize code that uses 
supports_cx8() function call by removing the actual reference to 
_supports_cx8 variable as well as eliminating code for the 
!supports_cx8() case.
Webrev is here: http://cr.openjdk.java.net/~bpittore/8014911/webrev.00/

thanks,
bill