Revert "Modified darwin_stop_world.c to use an arm_unified_thread_state_t struct to"

This reverts commit b45b5347a6e80ebc346195e6432167586d59b97b.
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.