Motivation for this change

I needed to submit a patch to GCC and they want confirmation of no regressions in the testsuite, and so I ended up using this expression with nix-build (parts specific to GCC trunk & my own patches omitted):

with import <nixpkgs>{};
gcc9.cc.overrideAttrs (gcc: {
    nativeBuildInputs = (gcc.nativeBuildInputs or []) ++ [ expect dejagnu ];
    builder = ./builder.sh; # with the changes in this PR
    doCheck = true;
    checkFlags = "-k";
})

Sadly, not all tests pass (it's unclear to me how many of those are broken upstream), so doCheck = true can't be turned on by default yet, but:

• expect and dejagnu dependencies are needed to run the test suite in the first place
• the builder.sh change allows crt{1,i}.o to be found for linking executable tests

Overall, this did work well enough for me to be able to confirm the lack of regressions (by building GCC trunk and GCC trunk + my patch, and comparing the .sum files in the resulting build dirs).

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 nix-review --run "nix-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.