Add crosscompile config files for 32, 64-bit Windows and 32-bit Linux

In order to have a linux machine build a windows-binary version of dav1d, this file must be present and used with --cross-file= in meson.

This was discovered in trying to update HandBrake to use ffmpeg with libdav1d and compile for a windows host/target: https://github.com/HandBrake/HandBrake/pull/1864/files#r314957654

This merge request also now contains crossfiles for 32-bit windows and linux.

The crossfiles added were obtained by inspecting the CI images's /opt/crossfiles directory.