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

EXTERNALLY_VISIBLE. 5% size reduction of libbusybox.so

coreutils/wc.c

@@ -68,7 +68,7 @@ enum {
 	WC_LENGTH	= 3
 };
 
-int wc_main(int argc, char **argv);
+int wc_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int wc_main(int argc, char **argv)
 {
 	FILE *fp;