MinGW (from mingw.org) is really old, so stop pretending we support it.
Just support MinGW-w64, which works also works with 32bit Windows,
despite it's misleading name.
PR-URL: https://github.com/libuv/libuv/pull/969
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>