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

EXTERNALLY_VISIBLE. 5% size reduction of libbusybox.so

coreutils/tail.c

@@ -79,7 +79,7 @@ static unsigned eat_num(const char *p)
 	return xatou_sfx(p, tail_suffixes);
 }
 
-int tail_main(int argc, char **argv);
+int tail_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int tail_main(int argc, char **argv)
 {
 	unsigned count = 10;