gcc8,gcc9: fix ctypes on NetBSD

[alyssais]

Motivation for this change

This patch was applied to gcc7 in aab8c7b ("netbsd: add cross target"),
but it hasn't been brought forward to newer compilers that have the
same problem.

GCC 6 and (probably) GCC 4.9 also have the issue, but the patch
doesn't apply cleanly to them so I'm leaving them alone for now.

GCC 10, our current default, appears to have finally fixed this.

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.

[sternenseemann]

SGTM.