@fpotter's suggestions, mostly

* Ended up going with a method that returned an (NSTask *) rather than one
  that modifies one via parameter. It seemed that either way, I'd have to
  remember to call the parent impl when it came to the two iOS test runners
  sharing behavior
* Added a property of type cpu_type_t. This actually ended up being a way for
  the two iOS test runners to not have to call the parent implementation.
  The two just expect [self cpuType] to return the correct arch, which is done
  by the parent.
* In CreateTask...(), passing an arch of CPU_TYPE_ANY doesn't set an arch.