6781583: Hotspot build fails on linux 64 bit platform with gcc 4.3.2

Fixed the wrong cast between types since more restrictions are imposed by gcc 4.3.2

Reviewed-by: jcoomes, acorn, phh, never

hotspot/src/share/vm/oops/oopsHierarchy.hpp

@@ -126,8 +126,11 @@ public:
   operator jobject () const           { return (jobject)obj(); }
   // from javaClasses.cpp
   operator JavaThread* () const       { return (JavaThread*)obj(); }
+
+#ifndef _LP64
   // from jvm.cpp
   operator jlong* () const            { return (jlong*)obj(); }
+#endif
 
   // from parNewGeneration and other things that want to get to the end of
   // an oop for stuff (like constMethodKlass.cpp, objArrayKlass.cpp)