[AVR] Add AVRMCTargetDesc.cpp

Summary:
This adds the AVRMCTargetDesc file in tree. It allows creation of the
core classes used in the backend.

Reviewers: arsenm, kparzysz

Subscribers: wdng, beanz, mgorny

Differential Revision: https://reviews.llvm.org/D25023

llvm-svn: 282597