llvm: ability to specify target cpu and features

To an extent, this addresses issues #27 and #12. ```python model = lleaves.Model(model_file=model_str_filename) model.compile( cache=lleaves_cache_filename, target_cpu='', # do not target a specific arch type target_cpu_features='+64bit,+avx,+avx2,+sse4.1', # enable these flags ) ``` per [`create_target_machine`](https://llvmlite.readthedocs.io/en/latest/user-guide/binding/target-information.html#llvmlite.binding.Target.create_target_machine), this will create a compilation target which does not target a specific cpu, and enables `avx`, `avx2` and `sse4.1` instruction sets