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
Maybe as a default for when `BUILD_OS` contains `CYGWIN`? I commonly build 32-bit Julia from 64-bit Cygwin, but wouldn't be opposed to setting XC_HOST by default for same-bitness - as long as it's easily overridable.