RFC: Document Cygwin-to-MinGW cross compilation

cygwin and msys-target compilers are incapable of building libuv, but can build everything else - saves user time and frustration to check for them up front and error early. ref #8488 and #6234
right, then you could do something like set `ARCH=i686` to switch builds, so that it doesn't look much like a cross-compile in the end