add -fvisibility=hidden to CC flags, mark XXX_main functions

EXTERNALLY_VISIBLE. 5% size reduction of libbusybox.so

networking/ifupdown.c

@@ -1133,7 +1133,7 @@ static llist_t *read_iface_state(void)
 }
 
 
-int ifupdown_main(int argc, char **argv);
+int ifupdown_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int ifupdown_main(int argc, char **argv)
 {
 	int (*cmds)(struct interface_defn_t *) = NULL;