[RFC] Multi-OS support

Change directory structure for multi-OS. Directories are now:

```
linux-x86
linux-x64
osx-x64
```

The `osx` directory is necessary because otherwise the bot-ci script will fail trying to perform a sparse Git checkout ("directory doesn't exist"). ~~OS X is not enabled in bot-ci yet, but it will be at some point, so we might as well create the directory now.~~ I only renamed the other directories because of consistency (and I think it looks nicer), but I can change that, of course.

~~At this point, a single commit to `bot-ci` to enable OS X will be enough, and OS X dependencies will be built, pushed here, and used in `neovim/neovim`.~~

Ref neovim/bot-ci#36, neovim/neovim#1538.