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

EXTERNALLY_VISIBLE. 5% size reduction of libbusybox.so

coreutils/du.c

@@ -140,7 +140,7 @@ static unsigned long du(const char *filename)
 	return sum;
 }
 
-int du_main(int argc, char **argv);
+int du_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int du_main(int argc, char **argv)
 {
 	unsigned long total;