Use the proper autoconf macro for checking byte order.

The BYTEORDER_IS_BIG_ENDIAN macro is defined Objects/unicodeobject.c,
thus the code was always emitting the little-endian machine format codes.

This should fix the failure seen on the SPARC buildbot.

Modules/arraymodule.c

@@ -1526,7 +1526,7 @@ static const struct mformatdescr {
 static enum machine_format_code
 typecode_to_mformat_code(int typecode)
 {
-#ifdef BYTEORDER_IS_BIG_ENDIAN
+#ifdef WORDS_BIGENDIAN
 	const int is_big_endian = 1;
 #else
 	const int is_big_endian = 0;