sim: ppc: stub out sim_memory_map

Not clear how to implement this in the ppc-specific sim, so just
stub it out.  This is as good as it was previously.

sim/ppc/ChangeLog

@@ -1,3 +1,7 @@
+2021-01-08  Mike Frysinger  <vapier@gentoo.org>
+
+	* sim_calls.c (sim_memory_map): Define.
+
 2021-01-04  Mike Frysinger  <vapier@gentoo.org>
 
 	* gen-icache.c, igen.c: Include stdlib.h.

sim/ppc/sim_calls.c

@@ -265,6 +265,12 @@ sim_complete_command (SIM_DESC sd, const char *text, const char *word)
   return NULL;
 }
 
+char *
+sim_memory_map (SIM_DESC sd)
+{
+  return NULL;
+}
+
 /* Polling, if required */
 
 void