GC_stack_range_for iOS 32/64-bit code refactoring (Darwin/arm)

* darwin_stop_world.c (kCFCoreFoundationVersionNumber_iOS_8_0): Move
definition out of GC_stack_range_for.
* darwin_stop_world.c (GC_stack_range_for): Refine comment and abort
message; eliminate code duplication regarding thread_get_state call.