clang, cc-wrapper: Move --gcc-toolchain logic into CC wrapper

[Ericson2314]

Motivation for this change

Take 2, after #94582 had to be reverted.

This reverts commit ac03cfa.

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.

[Ericson2314]

@ofborg build tests.cc-multilib-clang

[matthewbauer]

@Ericson2314 It would be nice to get rid of gccForLibs top-level. It looks like swift is still using it unfortunately.

[Ericson2314]

@matthewbauer Uh oh, I tried locally, and the clang multi tests failed. I think we need to chop up useGccForLibs into the cxx and everything-else cases to do this right. We might also figure out what about the -B and -L is really needed in addition to --gcc-toolchain.

[Ericson2314]

Oh, the tests were already broken. Nevermind then.