Support: correct Windows normalisation

If the environment is unknown and no object file is provided, then assume an
"MSVC" environment, otherwise, set the environment to the object file format.

In the case that we have a known environment but a non-native file format for
Windows (COFF) which is used for MCJIT, then append the custom file format to
the triple as an additional component.

This fixes the MCJIT tests on Windows.