6953952: collectedHeap.cpp should use #ifdef _LP64 not LP64

Summary: Changed LP64 to _LP64 in collectedHeap.cpp.
Reviewed-by: kvn, jcoomes

src/share/vm/gc_interface/collectedHeap.cpp

@@ -309,7 +309,7 @@ void CollectedHeap::fill_with_objects(HeapWord* start, size_t words, bool zap)
   DEBUG_ONLY(fill_args_check(start, words);)
   HandleMark hm;  // Free handles before leaving.
 
-#ifdef LP64
+#ifdef _LP64
   // A single array can fill ~8G, so multiple objects are needed only in 64-bit.
   // First fill with arrays, ensuring that any remaining space is big enough to
   // fill.  The remainder is filled with a single object.