Adding target support for Motorola 680x0

Hi!

There is currently a submission on Phabricator to add target support
for Motorola 680x0 with currently three open PRs:

https://reviews.llvm.org/D50858
https://reviews.llvm.org/D50856
https://reviews.llvm.org/D50784

Is there anyone who could help review the changes? Having target support
for M680x0 in LLVM is dearly anticipated by the Debian and NetBSD guys
who still have a fully working port for Motorola 680x0, so it would be
great if the groundwork for that could be laid.

I apologize for poking the mailing list for that but I am just too
excited about these patch series.

Adrian