sim: erc32/m32c/rl78: add sim_memory_map stub for gdb

These ports don't use the common sim core, so they weren't providing
a sim_memory_map for gdb, so they failed to link with the new memory
map logic added for the sim.  Add stubs to fix.

sim/rl78/gdb-if.c

@@ -603,3 +603,9 @@ sim_complete_command (SIM_DESC sd, const char *text, const char *word)
 {
     return NULL;
 }
+
+char *
+sim_memory_map (SIM_DESC sd)
+{
+  return NULL;
+}