Clean up fourcc!() docs

Cleans up a few issues with `fourcc`:
* Corrects the endianness in the docs example
* Removes `#[cfg(not(test))]` (bors might not build this on Windows. If the build fails, I'll re-add it)
* Adds a FIXME referencing the LLVM assert issue we encountered with bors builds on Windows (Same error as #10872)