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

EXTERNALLY_VISIBLE. 5% size reduction of libbusybox.so

networking/nc.c

@@ -22,7 +22,7 @@ static void timeout(int signum)
 	bb_error_msg_and_die("timed out");
 }
 
-int nc_main(int argc, char **argv);
+int nc_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int nc_main(int argc, char **argv)
 {
 	/* sfd sits _here_ only because of "repeat" option (-l -l). */