sim/rx: define sim_memory_map

The rx simulator doesn't define sim_memory_map and so fails to link
with GDB.  Define it now to return NULL, this can be extended later to
return an actual memory map if anyone wants this functionality.

sim/rx/ChangeLog:

	* gdb-if.c (sim_memory_map): New function.

sim/rx/gdb-if.c

@@ -854,3 +854,11 @@ sim_complete_command (SIM_DESC sd, const char *text, const char *word)
 {
   return NULL;
 }
+
+/* Stub this out for now.  */
+
+char *
+sim_memory_map (SIM_DESC sd)
+{
+  return NULL;
+}