[20.09] retroarch/cores: unmark broken

[Omoroca13]

Motivation for this change

in #101736 nearly all retroarch cores got marked as broken.
As far as I can tell on Hydra only mame2016 failed to build. (I am struggling too read it though so I might be missing something)

I tried my Cores with nixpkgs.config.allowBroken = true; and they seem to be working.

This pull request reverts the marking of all cores as broken and marks mame2016 as broken.

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"
• 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.

[SuperSandro2000]

I am guessing that the script is not smart enough to properly mark this package.

[SuperSandro2000]

Result of nixpkgs-review pr 102145 run on x86_64-darwin 1

1 package marked as broken and skipped:

• libretro.mame2016

[SuperSandro2000]

Result of nixpkgs-review pr 102145 run on x86_64-linux 1

1 package marked as broken and skipped:

• libretro.mame2016