Make liblzma produce the same output on both endiannesses.

Make liblzma produce the same output on both endiannesses.

Seems that it is a problem in some cases if the same
version of XZ Utils produces different output on different
endiannesses, so this commit fixes that problem. The output
will still vary between different XZ Utils versions, but I
cannot avoid that for now.

This commit bloatens the code on big endian systems by 1 KiB,
which should be OK since liblzma is bloated already. ;-)