Motivation for this change

Not strictly a neceesary package, but it seems useful for inspecting x86 binaries. I used this in the context of https://discourse.nixos.org/t/supported-instruction-sets/10755

Repology indicates that it's only in the AUR, so I'm not 100% sure if it suits our criterion for inclusion or not.
https://repology.org/project/elfx86exts/versions

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• X ] Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.