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

EXTERNALLY_VISIBLE. 5% size reduction of libbusybox.so

coreutils/dos2unix.c

@@ -78,7 +78,7 @@ static int convert(char *fn, int conv_type)
 	return 0;
 }
 
-int dos2unix_main(int argc, char **argv);
+int dos2unix_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int dos2unix_main(int argc, char **argv)
 {
 	int o, conv_type;