[RFC] Backend for Motorola 6800 series CPU (M68k)

Hi Simon!

Its awesome to see so much progress on this!

A very minor question - why is it called M680x0 and not M68K given
that's what the target arch/triple is and how its usually referred to?

"M680x0" is the name that Artyom chose when he originally worked on the backend
and I think it fits the naming scheme of LLVM nicely where the backend does not
necessarily have the same name as the GNU arch name.

For example, for s390x, the backend is called "SystemZ", for "riscv64" it's called
"RISCV", for "sparc" and "sparc64" it's called "Sparc" with capilization.

I think the advantage of this naming scheme is that one can easily tell whether you're
talking about the architecture or the backend itself which makes reading the code a
little easier.

PS: Sorry for not chiming in earlier, I'm currently reading the list in digest mode
    only but will change to full mode.