FYI, note that 3bb752736c4743754a6203293dcb62b52a1757c5 removed the relevant code, and mingw-w64 just imported a new version of widl that includes this change, so now Debian shouldn't need to patch widl in order to build it on exotic architectures.
When running on big endian, the binary IO in widl probably isn't correct, but one could probably expect the idl->h generation to work at least.