clang: only link if LLVMgold exists

buildEnv now errors with broken symlinks. Lots of things like this
still exist.

Discussion in #39038.

pkgs/development/compilers/llvm/6/clang/default.nix

@@ -53,7 +53,10 @@ let
     # Clang expects to find LLVMgold in its own prefix
     # Clang expects to find sanitizer libraries in its own prefix
     postInstall = ''
-      ln -sv ${llvm}/lib/LLVMgold.so $out/lib
+      if [ -e ${llvm}/lib/LLVMgold.so ]; then
+        ln -sv ${llvm}/lib/LLVMgold.so $out/lib
+      fi
+
       ln -sv ${llvm}/lib/clang/${release_version}/lib $out/lib/clang/${release_version}/
       ln -sv $out/bin/clang $out/bin/cpp