CI regression tests for DJGPP, Emscripten, and KallistiOS.

[AliceLR]

These platforms seem like the most plausible to add cross platform regression tests to CI for. DJGPP and Emscripten are needed in particular due to the number of hacks they require.

[AliceLR]

ALSO SDLWii, 3DS SDL (both), Darwin (all three variants), and Xcode would be very nice.

[AliceLR]

Emscripten, BlocksDS, and darwin-devel are in now. Might need to start merging the dkP-based tests since they are nearing the free runner limit.

edit: the setup-emsdk action and the MacPorts setup action should probably be forked at some point since they are both non-official.