LLVM-3.9: Fix RPATH in cross compile builds

This error was cause by multiple-outputs.sh not setting
NIX_CROSS_LDFLAGS

pkgs/development/compilers/llvm/3.9/llvm.nix

···
   preBuild = ''
     mkdir -p $out/
     ln -sv $PWD/lib $out
+  ''
+  + # This is a good candidate for using the `placeholder` primitive when it's released
+    # This should hopefully be unnecessary once
+    # https://github.com/NixOS/nixpkgs/pull/25047 is merged
+    stdenv.lib.optionalString (buildPlatform != hostPlatform && enableSharedLibraries) ''
+    export NIX_CROSS_LDFLAGS="-rpath $lib/lib -rpath $lib/lib64 $NIX_CROSS_LDFLAGS"
   '';
 
   cmakeFlags = with stdenv; [