libcxx

[ElvishJerricco]

This isn't necessary for WebGHC, but it's a nice-to-have that might kind of low hanging fruit. LLVM's libcxx does not currently build under the cross nixpkgs.

[ElvishJerricco]

#5 will make this worse. libcxx, libcxxabi, and libunwind all don't even build natively anymore. Considering we don't need C++ right now, I'm not too upset about that and still consider this very low priority