darwin.apple_sdk_11_0: use stdenv objc4

Stdenv on aarch64-darwin pulls in (bootstrap-stage4) objc4, unlike
x86_64. However derivations that otherwise depend on objc4 would use a
a different objc4 derivation on top of the final stdenv.

Because this library defines an LLVM module, having multiple instances
of it in the import path will interfere with builds.

pkgs/os-specific/darwin/apple-sdk-11.0/default.nix

···
     libcharset = callPackage ./libcharset.nix {};
     libunwind = callPackage ./libunwind.nix {};
     libnetwork = callPackage ./libnetwork.nix {};
-    objc4 = callPackage ./libobjc.nix {};
+    # Avoid introducing a new objc4 if stdenv already has one, to prevent
+    # conflicting LLVM modules.
+    objc4 = if stdenv ? objc4 then stdenv.objc4 else callPackage ./libobjc.nix {};
 
     # questionable aliases
     configd = pkgs.darwin.apple_sdk.frameworks.SystemConfiguration;

pkgs/stdenv/darwin/default.nix

···
         libc = pkgs.darwin.Libsystem;
         shellPackage = pkgs.bash;
         inherit bootstrapTools;
+      } // lib.optionalAttrs useAppleSDKLibs {
+        # This objc4 will be propagated to all builds using the final stdenv,
+        # and we shouldn't mix different builds, because they would be
+        # conflicting LLVM modules. Export it here so we can grab it later.
+        inherit (pkgs.darwin) objc4;
       };
 
       allowedRequisites = (with pkgs; [