[AVR] Allow specifying the CPU on the command line

Summary:
This tells clang about all of the different AVR microcontrollers.

It also adds code to define the correct preprocessor macros for each
device.

Reviewers: jroelofs, asl

Reviewed By: asl

Subscribers: asl, cfe-commits

Differential Revision: https://reviews.llvm.org/D28346

llvm-svn: 294177