Make x64 to use the RecordWrite stub. This is a step towards getting

src/ia32/macro-assembler-ia32.h

Index: src/ia32/macro-assembler-ia32.h
===================================================================
--- src/ia32/macro-assembler-ia32.h	(revision 8207)
+++ src/ia32/macro-assembler-ia32.h	(working copy)
@@ -78,9 +78,9 @@
     kFallThroughAtEnd
   };
 
-  // For page containing |object| mark region covering |addr| dirty.
-  // RememberedSetHelper only works if the object is not in new
-  // space.
+  // Record in the remembered set the fact that we have a pointer to new space
+  // at the address pointed to by the addr register.  Only works if addr is not
+  // in new space.
   void RememberedSetHelper(Register addr,
                            Register scratch,
                            SaveFPRegsMode save_fp,
@@ -138,10 +138,9 @@
 
   // Notify the garbage collector that we wrote a pointer into an object.
   // |object| is the object being stored into, |value| is the object being
-  // stored.  All registers are clobbered by the operation.  RecordWriteField
-  // filters out smis so it does not update the write barrier if the value is a
-  // smi.  The offset is the offset from the start of the object, not the offset
-  // from the tagged HeapObject pointer.  For use with FieldOperand(reg, off).
+  // stored.  value and scratch registers are clobbered by the operation.
+  // The offset is the offset from the start of the object, not the offset from
+  // the tagged HeapObject pointer.  For use with FieldOperand(reg, off).
   void RecordWriteField(
       Register object,
       int offset,