configure: Fix passing multiple target and host triples.

Previously we were actually overwriting `CFG_{HOST,TARGET,BUILD}` with `CFG_{HOST,TARGET,BUILD}_TRIPLE(S)` since configure tested for the legacy one by checking if it was empty which would never be the case. That meant it wouldn't split up multiple triples and just treat it as one long triple.

This pull also fixes the rules that were changed when librustuv was added to use the right CFG_ vars and removes the legacy flags.