Support AVR target features

[dylanmckay]

Once AVR-LLVM supports target features (see here), clang should support them too.

Ideally, without passing any extra arguments, clang would compile code for the lowest common denominator, assuming that there are no extra features and that only the core instruction set is supported. It should then be possible to pass flags such as clang --enable-avr-features=fmul,call or suchlike.

We could also do as AVR-GCC does - maintain a list of AVR devices and then map them to their supported features. For example, passing -mcpu=atmega328p will notify GCC to use all of the features supported by the Atmega328p. This is much easier but then we would need to update clang for every new AVR device, as opposed to updating it for every new feature.

We could even support both.

[4ntoine]

Few ideas:

• i'suggest avr-clang to be as compatible with avr-gcc on arguments as possible
• can we keep features-for-the-mcus-file separately to avoid recompiling clang every time new mcu is added? Then we will be able to edit the file and avr-clang will just parse it just in time.
• how to design it better? Are MCUs subtarget's or other LLVM/Clang entities? I'm not familiar with LLVM/Clang so deeply..

[4ntoine]

Add -mcpu to be passed to LLVM tools like llc

[4ntoine]

now -target-cpu avr2 is added for Clang compiler command-line by default.