Fallback to `ctypes.cdll` if `ctypes.windll` unavailable

On Windows, we may be using a native build of Python or a mingw/msys build. The
former exports `ctypes.windll`, the latter exports `cdll`. Previously the code
threw an exception when using the msys Python because it expected `windll` to
always be available on Windows.

Differential Revision: https://phabricator.haskell.org/D308