Regard endianness independent of a specific CPU type or OS

[68420948]

Regard endianess based on BYTE_ORDER instead of __SPARC or _AIX.

This makes sure endianness is handled correctly, at least with using GCC, without the need to fake a CPU type like proposed in https://github.com/trendmicro/tlsh/issues/131.

Picking endianness based on a specific OS seems not correct at all.