[AVR] Compute code model if one is not provided
The patch from r310028 fixed things to work with the new
LLVMTargetMachine constructor that came in on r309911.
However, the fix was partial since an object of type
CodeModel::Model must be passed to LLVMTargetMachine
(not one of Optional<CodeModel::Model>).
This patch fixes the problem in the same fashion that r309911
did for other machines: by checking if the passed optional
code model has a value and using CodeModel::Small if not.