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

EXTERNALLY_VISIBLE. 5% size reduction of libbusybox.so

networking/arping.c

@@ -227,7 +227,7 @@ static int recv_pack(unsigned char *buf, int len, struct sockaddr_ll *FROM)
 	return 1;
 }
 
-int arping_main(int argc, char **argv);
+int arping_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int arping_main(int argc, char **argv)
 {
 	const char *device = "eth0";