UDF (Undefined) instructions shouldn't advance PC
Unpredictable errors shouldn't advance PC
Flag tests to not run on real hardware

* Be careful to not set the SP to an invalid value since the MRI debug
  monitor used for validation has to use the stack as well for entering
  and leaving the debug monitor exception.
* Also be careful to not set PC to invalid values.
* Tests which expect PINKYSIM_STEP_UNPREDICTABLE and
  PINKYSIM_STEP_UNSUPPORTED to be returned aren't appropriate.
* Some things like CONTROL and PRIMASK are just ignored by the
  simulator but not real hardware.
* Don't try to change the value of the IPSR register on real hardware.

I also moved the INITIAL_PC value up half way in the first RAM bank of
the LPC1768.  Before it would collide with globals used by the MRI
debug monitor.
Fix search/replace bug in last commit