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

EXTERNALLY_VISIBLE. 5% size reduction of libbusybox.so

coreutils/od.c

@@ -156,7 +156,7 @@ static const char od_o2si[] ALIGN1 = {
 	0xb, 1, 8, 9,
 };
 
-int od_main(int argc, char **argv);
+int od_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int od_main(int argc, char **argv)
 {
 	int ch;