[RFC] More misc. platform cleanup

This was mostly straightforward, but I do have one question.

I saw that `version.c` has `-ebcdic` hardcoded in, presumably from when EBCDIC support was removed a while ago. Given that the flag's existence might falsely imply that EBCDIC support can even be compiled in, can't it just be removed?

edit: One more thing, should https://github.com/Pyrohh/neovim/commit/7352bec320744f029c157277ed7754e44c713efb be moved to a separate pull request?
Yes. I think we should keep `gui.txt` filename the same, but the content itself will probably be mostly replaced.

msgpack-rpc details should not be mentioned in too many places outside of the rpc doc itself, I doubt most users love it as much as we do :)