tmon: fix missing libgcc_s.so required by pthreads

[lopsided98]

Motivation for this change

Tmon crashes on exit when cross-compiled, with this error:

libgcc_s.so.1 must be installed for pthread_cancel to work

This is caused by what appears to be a bug with where libgcc_s.so is placed, see: #40797

Things done

This PR fixes this issue by manually linking to libgcc_s as is done in a number of other packages.

• 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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