Fix assimp build on uncommon architectures

This adapts patch d5bd7009a609a52ced576917d24e09fd336e389c from upstream.

The build should not error out when it encounters an architecture that
has not been explicitly included. The truly annoying part of the error
was that it *only* affected how a debug string was constructed.

Using the byte order as provided by the build system is much nicer and
hopefully cleaner.

Change-Id: I62c0e77934cc37ea9f60c8b3175bac79529088fa
Done-with: Mika Boström <mika.bostrom@nomovok.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>