[AVR] Add the machine code backend

Summary:
This adds the AVR machine code backend (`AVRAsmBackend.cpp`). This will
allow us to generate machine code from assembled AVR instructions.

Reviewers: arsenm, kparzysz

Subscribers: modocache, japaric, wdng, beanz, mgorny

Differential Revision: https://reviews.llvm.org/D25029

llvm-svn: 283297