Passing `attribute_path` to `nixpkgs_cc_configure` breaks on Darwin

[benradf]

Describe the bug See this comment for details, but essentially by passing attribute_path we go down the ccTypeExpression path which overrides the custom nix expression for Darwin which handles the different tool paths.

Expected behavior If there's a sensible way to do so, it would be good to be able to use attribute_path in the custom nix expression for Darwin. Failing that, we should at least error out in nixpkgs_cc_configure so the user doesn't get confusing messages about /bin/false and missing tools when linking.

Additional context #368

[borancar]

This is also a problem on Linux, requiring a similar fix. The problem is that putting a gcc7/8/9/10 etc... just pulls in gcc, without bintools, so strip and link end up being /bin/false.