Use ARM_THREAD_STATE32 as flavor in call to thread_get_state() on iOS 64-bit

Use the ARM_THREAD_STATE32 flavor in the call to thread_get_state() when running on 64-bit iOS 7+. If we don't iOS will assume we pass it an arm_unified_thread_state_t while we actually pass it an arm_thread_state_t
which is a lot smaller. Without this fix thread_get_state() will corrupt the stack and the app will crash.

Please note that this only seem to be a problem if the app is compiled using Xcode6. We don't see this with apps built using Xcode5.