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

EXTERNALLY_VISIBLE. 5% size reduction of libbusybox.so

coreutils/head.c

@@ -31,7 +31,7 @@ static const struct suffix_mult head_suffixes[] = {
 
 static const char header_fmt_str[] ALIGN1 = "\n==> %s <==\n";
 
-int head_main(int argc, char **argv);
+int head_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int head_main(int argc, char **argv)
 {
 	unsigned long count = 10;