Fix 64 bit big endian code path in decoder.

We were always taking little endian code path.

Fix originally from Niko Tyni of the Debian project for the Sereal
project. See also:
https://github.com/Sereal/Sereal/commit/dd977ade7cdc15718bcebf95538edae9281ba3e1
Fix broken C comment in big-endian code path
Detect endianness correctly on s390x (Z/Os) and Sparc machines

Ported from the Sereal fork of csnappy.
Tweak authors/modifiers list to match Sereal version