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).