entropy throws and error during build on GHC-9.x for the JS backend in nixpkgs.

See this ticket: https://github.com/NixOS/nixpkgs/issues/337996

for the whole story.

Essentially what is happening is the `Setup.hs` file uses inline assembly to call `rdrand` which is not supported for JS because emscripten does not support it.

So the fix is to either write that assembly in portable C/C++ or write some javascript that does the same thing and use and `ifdef` to catch when `entropy` is being cross-compiled.

I'm more than happy to write the js if you need it, but I wanted to defer to the author of the library to make the decision on how best to proceed.