Enabling a target not in `SUPPORTED_PLATFORM_TRIPLES` silently generates BUILD files that won't work

[bsilver8192]

I've got a few new Linux triples which aren't in SUPPORTED_PLATFORM_TRIPLES yet (PR to add them coming soon). It was tricky to figure out why a cfg(unix) dependency didn't work for them at first.

I think a warning or error when running raze with unsupported triples in targets would be helpful. Thoughts?

[dfreese]

If a user is explicitly specifying target triples, an error if we don't recognize that triple would make sense. I'd be supportive of that.

There's the bigger problem of keeping raze's triples in-sync with rust, and that's less obvious to me.