Consider cross-compilation in TH mode

[edsko]

We need to consider what the workflow will look like; how does the user start a build on their host machine for a particular target machine, and importantly, how do those flags then make it to the TH splice? We could require users to do set this up themselves (as long as we accept clang args, #341, that's at least possible), but it would be good to provide some out-of-the-box support.

[phadej]

Cross compilation doesn't really make sense for the current TH setup. TH is always run on the target architecture, so for now the TH story is actually quite straight forward.