Modified darwin_stop_world.c to use an arm_unified_thread_state_t struct to

Modified darwin_stop_world.c to use an arm_unified_thread_state_t struct to
store a thread's state when compiling against the iOS 7/8 SDK. Without this
patch GC_stack_range_for() crashes when running a 32-bit app on iOS 64-bit.
The old code passed an arm_thread_state_t and ARM_THREAD_STATE to
thread_get_state(). ARM_THREAD_STATE is the same as ARM_UNIFIED_THREAD_STATE
on iOS 7/8 and thread_get_state() actually expects an
arm_unified_thread_state_t. On iOS 32-bit it looks like thread_get_state()
only touches the first bytes corresponding to the size of arm_thread_state_t
so no crash there. On iOS 64-bit however it seems thread_get_state() writes to
the full arm_unified_thread_state_t which meant it would overflow the stack
allocated struct passed to it and mess up other values on the stack leading to
a crash later on.