Commit: Export AArch64 sum's single step function

[Nick Clifton]

Hi Mike,

> this is already possible with the common sim API -- sim_resume() has a
> "step" argument specifically to do single stepping.

Hmm, OK, but how is this single stepping communicated to the target simulator ?

As far as I can see the AArch64 simulator does not support single stepping 
(unless the host has inserted breakpoint instructions into the simulated instruction
space).  I assume that this is a bug in the aarch64 simulator code, but what exactly
is wrong ?

Hmm, I see that some other target simulators implement a sim_resume() function of
their own.  Perhaps this is the problem - the aarch64 sim should do the same ?

Cheers
  Nick