[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.

llvm-svn: 310200