[AVR] Add the assembly instruction printer

Summary:
This change adds the AVR assembly instruction printer.

No tests are included in this patch. I have left them downstream so we can
add them once `llc` successfully runs (there's very few components left
to upstream until this).

Reviewers: arsenm, kparzysz

Subscribers: wdng, beanz, mgorny

Differential Revision: https://reviews.llvm.org/D25028

llvm-svn: 282854