Replace OSAtomicIncrement/Decrement32Barrier with OSAtomicAdd32Barrier

fix issue with running on iOS 7.0 devices with application built against iOS 7.1 sdk.  OSAtomicIncrement32Barrier and OSAtomicDecrement32Barrier cause a dyld: lazy symbol binding failed: Symbol not found: _OSAtomicIncrement32Barrier error in this case on iOS 7.0.  resolves #1223