[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?
Oops I wasn't clear. I meant the removal of the `gui_*.txt` docs, to be replaced by a unified doc on how an nvim instance interacts via msgpack-rpc with GUI clients.