pkgsLLVM.stdenv: use lld again

PR #122778 allowed the linker being chosen independently from useLLVM
which also affected pkgsLLVM where we were relying on this behavior.
For platform sets assembled from scratch useLLVM still implies
linker == "lld", however in the case of pkgsLLVM we update the current
platform via the set update operator which means that `linker` won't
be re-evaluated. Using ld.bfd with pkgsLLVM is okay to a certain extent,
but with C++ things begin to break.

We fix this by setting linker explicitly.

pkgs/top-level/stage.nix

@@ -169,6 +169,7 @@ let
       # so we don't need to check hostPlatform != buildPlatform.
       crossSystem = stdenv.hostPlatform // {
         useLLVM = true;
+        linker = "lld";
       };
     };