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

EXTERNALLY_VISIBLE. 5% size reduction of libbusybox.so

coreutils/ln.c

@@ -22,7 +22,7 @@
 #define LN_BACKUP           8
 #define LN_SUFFIX           16
 
-int ln_main(int argc, char **argv);
+int ln_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int ln_main(int argc, char **argv)
 {
 	int status = EXIT_SUCCESS;