windows: mingw32 gcc doesn't define endianness macros, so just assume little.

Specifically, at least Ubuntu's i586-mingw32msvc-gcc doesn't supply
__LITTLE_ENDIAN__ and friends. And as it's safe enough to assume that
Windows is only little-endian, then it's defined unconditionally,
instead of duplicating detection logic in py/mpconfig.h (or adding
windows-specific defines to it).

windows/mpconfigport.h

@@ -88,6 +88,10 @@ typedef unsigned int mp_uint_t; // must be pointer size
 
 #define BYTES_PER_WORD sizeof(mp_int_t)
 
+// Just assume Windows is little-endian - mingw32 gcc doesn't
+// define standard endianness macros.
+#define MP_ENDIANNESS_LITTLE (1)
+
 typedef void *machine_ptr_t; // must be of pointer size
 typedef const void *machine_const_ptr_t; // must be of pointer size