Sense int8 or int8Shift

[jerinphilip]

Routing/dispatch precedence can be preconfigured in source. There's already platform detection lying around.

My problem with the existing setup is the easy way for me to maintain a continuous integration/test setup (python) for #79 now requires maintaining two sets of configuration files or detecting platform and switching at the client.

At least for native, compiled marian targetting ARM or Intel should be aware which is the best path to take.
marian-nmt#762 appears to have omitted Shift path entirely.