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
