sim: keep track of program environment strings

We've been passing the environment strings to sim_create_inferior,
but most ports don't do anything with them.  A few will use ad-hoc
logic to stuff the stack for user-mode programs, but that's it.

Let's formalize this across the board by storing the strings in the
normal sim state.  This will allow (in future commits) supporting
more functionality in the run interface, and to unify some of the
libgloss syscalls.

sim/example-synacor/interp.c

@@ -174,5 +174,11 @@ sim_create_inferior (SIM_DESC sd, struct bfd *abfd,
       STATE_PROG_ARGV (sd) = dupargv (argv);
     }
 
+  if (STATE_PROG_ENVP (sd) != env)
+    {
+      freeargv (STATE_PROG_ENVP (sd));
+      STATE_PROG_ENVP (sd) = dupargv (env);
+    }
+
   return SIM_RC_OK;
 }