{cc,bintools}-wrapper: Some fixes

[Ericson2314]

Motivation for this change

1. This means we can freely keep the comments up to date without the penalty of a mass rebuild.

2. This will unbreak firefox and a few other packages which try to grab some of the libcxx flags.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built stdenv 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.

[vcunat]

I'm mainly curious... this dash is some kind of special operator? I can't find this one in man bash.

[veprbl]

@vcunat See https://wiki.bash-hackers.org/syntax/pe#use_a_default_value

If you omit the : (colon), like shown in the second form, the default value is only used when the parameter was unset, not when it was empty.

[vcunat]

Thanks. Now I also read that man bash part again – more properly – and I see it there as well 🤦

[vcunat]

Let's have this in the current staging-next iteration. The risks seem low enough and otherwise we'd have to work around those problems in some other way.