Support fat binary Windows on Ruby 3.1 (x64-mingw-ucrt)

[johnnyshields]

Fixes #237

As of Ruby 3.1, the Ruby Windows Installer is now releasing its platform as x64-mingw-ucrt. Refer to https://github.com/ruby/setup-ruby/issues/193

Compare Ruby 3.0 vs. 3.1 on Windows:

ruby 3.0.0p0 (2020-12-25 revision 95aff21468) [x64-mingw32]
ruby 3.1.1p18 (2022-02-18 revision 53f5fc4236) [x64-mingw-ucrt]

This patch adds x64-mingw-ucrt to the cross-compilation, so that a fat binary will be created for x64-mingw-ucrt platform.