[sdl2] [ci] add an os/2-emx workflow using cross-os2emx? #16403

Asking @madebr -- I couldn't do myself.. Example workflows in other projects:
https://github.com/libxmp/libxmp/blob/master/.github/workflows/regression.yaml#L465-L482
https://github.com/Mindwerks/wildmidi/blob/master/.github/workflows/cross.yml#L67-L82