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

EXTERNALLY_VISIBLE. 5% size reduction of libbusybox.so

init/init.c

@@ -882,7 +882,7 @@ static void reload_signal(int sig ATTRIBUTE_UNUSED)
 }
 #endif  /* FEATURE_USE_INITTAB */
 
-int init_main(int argc, char **argv);
+int init_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int init_main(int argc, char **argv)
 {
 	struct init_action *a;