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

EXTERNALLY_VISIBLE. 5% size reduction of libbusybox.so

util-linux/mdev.c

@@ -287,7 +287,7 @@ static void load_firmware(const char *const firmware, const char *const sysfs_pa
 	}
 }
 
-int mdev_main(int argc, char **argv);
+int mdev_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int mdev_main(int argc, char **argv)
 {
 	char *action;