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

EXTERNALLY_VISIBLE. 5% size reduction of libbusybox.so

networking/arp.c

@@ -434,7 +434,7 @@ static int arp_show(char *name)
 	return 0;
 }
 
-int arp_main(int argc, char **argv);
+int arp_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int arp_main(int argc, char **argv)
 {
 	char *hw_type;