Psmobile small fixes

Applied fixes described in http://community.monogame.net/t/spritebatch-cs-effect-cs-this-does-not-appear-to-be-a-monogame-mgfx-file/1070/2

Small fix to be able to use XNA XNBs without having to "manually edit them as stated in http://chainedchaos31.tumblr.com/post/97557910457/playing-audio-on-the-playstation-vita-via-monogame

What is the goal of this alternate cache block?  What is it fixing?