[RFC] Simplify MinGW canadian crosses

[DJ Delorie]

> Corinna is trying to generate --host=i686-mingw32 tools, with a
> different --target.  This requires at least a --target=i686-mingw32
> compiler coming from elsewhere.  That compiler can build the
> --host=i686-mingw32 libraries, and usually should.

Yes.  So?  We build that compiler from the same tree (as a simple
cross), so it's still the same problem.  We still have to build those
libraries somehow, we still need to have them in source control, etc.
Our build farm likes to see a monolithic source tree, and we already
support building cygwin out of that tree, building mingw out of it is
a minor change.

(actually, we build four compilers out of that tree:
	linux-linux-linux
	linux-linux-mingw
	linux-linux-arm
	linux-mingw-arm)