Use the proper autoconf macro for checking byte order.
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.

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator