core/pkgs/by-name/li/libunwind/packages.nix

{ ... }:
res: pkgs: super:

with pkgs; {
  libunwind =
    if stdenv.isDarwin then
      darwin.libunwind
    else if stdenv.hostPlatform.system == "riscv32-linux" then
      llvmPackages_14.libunwind
    else
      callPackage ./. { };
}