cygwin: init as a target toolchain

The old cygwin support used -pc-windows-cygnus as the config.  This is
supported by LLVM, but not by GNU. This will change it to -pc-cygwin,
which is more generally supported.

Because the kernel is now 'cygwin' rather than 'windows', isWindows will
return false. There are lots of different reasons isWindows is used in
nixpkgs, but in my experience they often have to do with posix
compatibility and don't apply to cygwin.

Co-authored-by: Brian McKenna <brian@brianmckenna.org>

lib/tests/systems.nix

@@ -49,6 +49,7 @@ lib.runTests (
       ++ illumos
       ++ wasi
       ++ windows
+      ++ cygwin
       ++ embedded
       ++ mmix
       ++ js
@@ -202,8 +203,6 @@ lib.runTests (
       "x86_64-openbsd"
     ];
     testwindows = mseteq windows [
-      "i686-cygwin"
-      "x86_64-cygwin"
       "aarch64-windows"
       "i686-windows"
       "x86_64-windows"