sim: callback: expose argv & environ

Pass the existing strings data to the callbacks so that common
libgloss syscalls can be implemented (which we'll do shortly).

sim/common/callback.c

@@ -770,6 +770,8 @@ host_callback default_callback =
 
   /* Defaults expected to be overridden at initialization, where needed.  */
   BFD_ENDIAN_UNKNOWN, /* target_endian */
+  NULL, /* argv */
+  NULL, /* envp */
   4, /* target_sizeof_int */
 
   HOST_CALLBACK_MAGIC,