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

EXTERNALLY_VISIBLE. 5% size reduction of libbusybox.so

util-linux/hexdump.c

@@ -54,7 +54,7 @@ static const struct suffix_mult suffixes[] = {
 	{ }
 };
 
-int hexdump_main(int argc, char **argv);
+int hexdump_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int hexdump_main(int argc, char **argv)
 {
 	const char *p;