Currently we can not place toolbaritems on the left side, on default. It's possible to write a renderer for the ContentPage on ios.
See https://timeyoutake.it/2016/01/02/creating-a-left-toolbaritem-in-xamarin-forms/ for an example.
But I find this code not very beautiful and it feels a bit tricky to use it. It would be nice if we can place toolbaritems on the left on default.
I talked with John Miller from Xamarin about this and he said that it does not make sense on other platforms than ios. I can agree on that, but I don't like the current solution either.
I hope that Xamarin comes with a solution on this, since it's a very common situation. (Cancel ------- Save for example).