Summary:
Fix some `-Werror` failures and work around a
bug in the `x86` version of `mingw-w64-crt`'s libraries.

The bump in the `win32` submodule is required for this.

Test Plan: ./validate

Reviewers: austin, bgamari, erikd, simonmar

Reviewed By: simonmar

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3362