tcl: link with libgcc_s

[pbogdan]

Motivation for this change

I'm not confident this is just the right condition to link to it only on platforms / environments where this is needed.

/cc @matthewbauer @dtzWill

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option 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/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

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[matthewbauer]

I guess it depends on how many of these kind of fixes are needed. If there are lots of libgcc_s that now need to be added, we probably need to figure out how to get things to pick up the "default" one.