Imported files and patches from GitHub user sushihangover's llvm-avr fork.

https://github.com/sushihangover/llvm-avr
Merged patches that I didn't see.
Updated AVR backend to master LLVM branch.
Split AVRTargetMachine and AVRSubtarget as they should be.
Modified AVR sources to use the new AVRTargetMachine and AVRSubtarget classes.
Added information about AVR backend to README.