[AVR] Further progress on ELF support
[AVR] Specified binary sizes of instructions
[AVR] Fixed segmentation fault and added LE and BE AvrAsmBackend
[AVR] Fixed endian issue with AVRCodeEmitter

Outputted code had the byte order swapped.
[AVR] Removed code from AVRMCCodeEmitter
[AVR] Set HWEncoding for registers in AVRRegisterInfo.td

Before this, all register encodings were assumed to be zero, and so every register access would read or write to r0.
[AVR] Fixed AVR `cli` and `sli` instructions

They were encoded as `Pseudo` instructions which don't exist physically. Obviously this is wrong because cli and sli are real instructions.
[AVR] Cleaned up AVRInstrFormats.td and AVRInstrInfo.td
[AVR] Documented ST(D) instructions
[AVR] Implemented code generation for STD instruction

This is a horrible, horrible hack.

Also made Pseudo instructions emit 0xBEEF values instead of NOPs.
This way it is more obvious that there is an error.