llvm-5: add extra 'python' output for opt-viewer tool

[dtzWill]

#33374 (review)

We don't really support the opt-viewer tool anyway,
but keep it around in case someone wants to play with it.

Don't pull python into the closure just for that reason, though.

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• 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 nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

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[vcunat]

Sounds OK to me.

[dtzWill]

We could alternatively delete it, but saving it to an extra output seems a bit kinder :).

Eventually we can try wrapping it or something to get it working, but mostly want to fix Darwin closure with LLVM 5.

[LnL7]

Looks good in principle, testing if this helps with the stdenv references.

[dtzWill]

@LnL7 we have a "success" regarding this resolving the references problem right?

I suppose there's no rush motivating merging this separately from #33374, but just checking :).

EDIT: Conflict resolution underway... done!