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

EXTERNALLY_VISIBLE. 5% size reduction of libbusybox.so

selinux/setfiles.c

@@ -486,7 +486,7 @@ static int process_one(char *name)
 	goto out;
 }
 
-int setfiles_main(int argc, char **argv);
+int setfiles_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int setfiles_main(int argc, char **argv)
 {
 	struct stat sb;