[AVR] Add assembly parser

Summary: This patch adds the AVRAsmParser library.

Reviewers: arsenm, kparzysz

Subscribers: wdng, beanz, mgorny, kparzysz, simoncook, jtbandes, llvm-commits

Differential Revision: https://reviews.llvm.org/D20046

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@282584 91177308-0d34-0410-b5e6-96231b3b80d8

Author: Dylan McKay