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

EXTERNALLY_VISIBLE. 5% size reduction of libbusybox.so

util-linux/fsck_minix.c

@@ -1249,7 +1249,7 @@ static void check2(void)
 void check2(void);
 #endif
 
-int fsck_minix_main(int argc, char **argv);
+int fsck_minix_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int fsck_minix_main(int argc, char **argv)
 {
 	struct termios tmp;