Fix iOS specs when run on the 64-bit simulator.

I noticed that the iOSSpecs target wouldn't build on 64-bit simulator destinations due to failures in the CDRExampleStateMap spec, because the default enum storage type isn't pointer-sized there. Explicitly setting the storage type fixes this. Also, this takes advantage of a feature that wasn't available until a few minutes ago when we supported building with the 5.0 SDK :smile: