stdenv: throw when using mold for targeting darwin

Mold does not support linking Mach-O, but still can be used on MacOS to
emit ELF.

Fixes: #338970

pkgs/stdenv/adapters.nix

@@ -261,7 +261,10 @@ rec {
     in
     overrideCC targetStdenv cc;
 
-  useMoldLinker = stdenv: let
+  useMoldLinker = stdenv:
+    if stdenv.targetPlatform.isDarwin
+    then throw "Mold can't be used to emit Mach-O (Darwin) binaries"
+    else let
       bintools = stdenv.cc.bintools.override {
         extraBuildCommands = ''
           wrap ${stdenv.cc.bintools.targetPrefix}ld.mold ${../build-support/bintools-wrapper/ld-wrapper.sh} ${pkgs.mold}/bin/ld.mold
@@ -280,7 +283,6 @@ rec {
         });
       });
 
-
   /* Modify a stdenv so that it builds binaries optimized specifically
      for the machine they are built on.