revert the changes to fix the problem in big-endian architectures

There is an endianness issue in pixman-fast-path.c. In the function bits_image_fetch_separable_convolution_affine we have this code:

#ifdef WORDS_BIGENDIAN
	buffer[k] = (satot << 0) | (srtot << 8) | (sgtot << 16) | (sbtot << 24);
#else
	buffer[k] = (satot << 24) | (srtot << 16) | (sgtot << 8) | (sbtot << 0);
#endif

This will write out the pixels as BGRA on big endian systems but obviously that's wrong. Pixel order should be ARGB on big endian systems so we don't need any #ifdef for big endian here at all. Instead, the code should be the same on little and big endian, i.e. it should be just this line instead of the 5 lines above:

buffer[k] = (satot << 24) | (srtot << 16) | (sgtot << 8) | (sbtot << 0);

Changing the code like this fixes librsvg test failures on big-endian architectures and the wrong colors that I get with pixman on my PowerPC system.