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

EXTERNALLY_VISIBLE. 5% size reduction of libbusybox.so

coreutils/cal.c

@@ -75,7 +75,7 @@ static char *build_row(char *p, unsigned *dp);
 #define	J_WEEK_LEN	(WEEK_LEN + 7)
 #define	HEAD_SEP	2		/* spaces between day headings */
 
-int cal_main(int argc, char **argv);
+int cal_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int cal_main(int argc, char **argv)
 {
 	struct tm *local_time;