sim: ppc: switch to libiberty environ.h

Drop our compat code and assume environ exists to simplify.
We did this for all other targets already, but ppc was missed.

sim/ppc/main.c

@@ -42,14 +42,14 @@
 #include <string.h>
 #include <errno.h>
 
+#include "environ.h"
+
 #if !defined(O_NONBLOCK) || !defined(F_GETFL) || !defined(F_SETFL)
 #undef WITH_STDIO
 #define WITH_STDIO DO_USE_STDIO
 #endif
 
 
-extern char **environ;
-
 static psim *simulation = NULL;