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/top-level/static.nix

@@ -25,7 +25,7 @@ self: super: let
   }; in stdenv // {
     mkDerivation = args: stdenv.mkDerivation (args // {
       NIX_CFLAGS_LINK = toString (args.NIX_CFLAGS_LINK or "")
-                      + optionalString stdenv.cc.isGNU " -static-libgcc";
+                      + optionalString (stdenv_.cc.isGNU or false) " -static-libgcc";
       nativeBuildInputs = (args.nativeBuildInputs or []) ++ [ (makeSetupHook {
         substitutions = {
           libsystem = "${stdenv.cc.libc}/lib/libSystem.B.dylib";