TemplateHaskell notoriously requires running host code even when cross-compiling. This is realised by telling GHC about a so called iserv via the -fexternal-interpreter flag. This iserv can either run on a physical or virtual machine matching the host platform or run via qemu (user) emulation. We should allow for the latter in a convenient way in nixpkgs. haskell.nix already supports this and relies on the nixpkgs cross infrastructure, so we can probably compare notes.