Exclude other OS folders from a OS relase

[dlangBugzillaToGithub]

Seb reported this on 2016-08-07T20:52:25Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=16361

Description

If one installs dmd on Linux via the installer script or downloads the release directly, he will the following folders:

- freebsd
- linux
- osx
- solaris
- windows

Imho that's pretty confusing to newbies (it was too at least) and we should exclude those nearly empty directories from the releases.
Also using the folder `linux` in a linux release is a bit pointless, but I expect that changing this might break a lot :/