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

EXTERNALLY_VISIBLE. 5% size reduction of libbusybox.so

util-linux/switch_root.c

@@ -64,7 +64,7 @@ static void delete_contents(const char *directory)
 	} else unlink(directory);
 }
 
-int switch_root_main(int argc, char **argv);
+int switch_root_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int switch_root_main(int argc, char **argv)
 {
 	char *newroot, *console = NULL;