Add cross bundle support

[seanybaggins]

Steps to test

nix bundle \
  --bundler 'github:seanybaggins/nix-bundle?ref=add-cross-bundle-support#toArxArm64Cross' \
  nixpkgs#pkgs.pkgsCross.aarch64-multiplatform.hello

Copy bin to device with aarch64 architecture. Then...

./hello-arx
Cannot stat /root: No such file or directory
Hello, world!

Local bundling

nix bundle \
  --bundler 'github:seanybaggins/nix-bundle?ref=add-cross-bundle-support' \
  nixpkgs#hello

 ./hello-arx
Hello, world!