Build system: delete half-baked Cygwin support

We only support building GHC on mys2 nowadays, see
https://ghc.haskell.org/trac/ghc/wiki/Building/Preparation/Windows.

This (partially) reverts various commits from a few years ago, among which:
  * 5775d514
    "Add OSTYPE build-system variable, and use it"
  * 3fb8c431
    "Fix building libgmp on cygwin"
  * cdbb4720
    "Fix cmd invocation by libffi cuild system on Windows 7 cygwin"
  * e8121501
    "Fix dblatex and xml* tool detection on Windows"

Reviewed by: austin, Phyx

Differential Revision: https://phabricator.haskell.org/D1155