Auto merge of #2870 - matklad:add-host-triple, r=alexcrichton

Refactor `host/target_triple` handling

This PR adds an explicit `host_triple` filed to `BuildConfig`. It allows to unify host and target handling and to reduce the number of accesses to `config.rustc_info()`. As a side effect, the explicit `target_triple` field of `Context` is gone because it can now be calculated from the `BuildConfig`.