wrapClangMulti: link include/c++ into gcc_multi_sysroot

The phoney gcc that we construct for multilib was missing the
`$out/include/c++` directory which `cc-wrapper` needs to pass as an
`-isystem` to `clang`.

Closes #221891

pkgs/development/compilers/llvm/multi.nix

@@ -28,7 +28,9 @@ let
 
 
     # includes
-    ln -s ${glibc_multi.dev}/include $out/
+    mkdir -p $out/include
+    ln -s ${glibc_multi.dev}/include/* $out/include
+    ln -s ${gcc64.cc}/include/c++ $out/include/c++
 
     # dynamic linkers
     mkdir -p $out/lib/32