8014911: Should use SUPPORTS_NATIVE_CX8 define to help C/C++ compiler elide blocks of code

If SUPPORTS_NATIVE_CX8 true then supports_cx8() function hard coded to return 'true'

Reviewed-by: kvn, twisti, dholmes

hotspot/src/share/vm/runtime/vm_version.hpp

@@ -78,7 +78,13 @@ class Abstract_VM_Version: AllStatic {
   static const char* jre_release_version();
 
   // does HW support an 8-byte compare-exchange operation?
-  static bool supports_cx8()  {return _supports_cx8;}
+  static bool supports_cx8()  {
+#ifdef SUPPORTS_NATIVE_CX8
+    return true;
+#else
+    return _supports_cx8;
+#endif
+  }
   // does HW support atomic get-and-set or atomic get-and-add?  Used
   // to guide intrinsification decisions for Unsafe atomic ops
   static bool supports_atomic_getset4()  {return _supports_atomic_getset4;}