treewide: Do a number of no-op cleanups for cross and darwin

I am taking the non-invasive parts of #110914 to hopefully help out with #111988.

In particular:

 - Use `lib.makeScopeWithSplicing` to make the `darwin` package set have
   a proper `callPackage`.

 - Adjust Darwin `stdenv`'s overlays keeping things from the previous
   stage to not stick around too much.

 - Expose `binutilsNoLibc` / `darwin.binutilsNoLibc` to hopefully get us
   closer to a unified LLVM and GCC bootstrap.

pkgs/development/interpreters/python/default.nix

@@ -69,6 +69,7 @@ with pkgs;
               recursivePthLoader
             ;
           };
+          extra = _: {};
           optionalExtensions = cond: as: if cond then as else [];
           python2Extension = import ../../../top-level/python2-packages.nix;
           extensions = lib.composeManyExtensions ((optionalExtensions (!self.isPy3k) [python2Extension]) ++ [ overrides ]);
@@ -77,6 +78,7 @@ with pkgs;
           pkgs.newScope
           otherSplices
           keep
+          extra
           (lib.extends extensions pythonPackagesFun))
         {
           overrides = packageOverrides;