[RFA] enable software single step on alpha-osf

[Andrew Cagney]

> So, to summarize: 
> 
>   1 - The change "+           stop_pc -= DECR_PC_AFTER_BREAK;" seems
>       to be going in the right direction.
> 
>   2 - However I should hold this change for now because you think I should
>       write the adjusted PC value back to the target, by adding something
>       like "write_pc_pid (stop_pc, ecs->ptid)"
> 
>       I will verify the impact of such a change, and report.
>     
>   3 - Assuming we get all issues in this RFA resolved, then I will start
>       looking at the addition of the software_singlestep flag.

I think the flag should be added as part of the change.  That way we're 
100% certain that bpstat_stop_status() isn't going to do a 
decr_pc_after_break.

Expressions like:

/* Pass TRUE if our reason for stopping is something other
    than hitting a breakpoint.  We do this by checking that
    1) stepping is going on and 2) we didn't hit a breakpoint
    in a signal handler without an intervening stop in
    sigtramp, which is detected by a new stack pointer value
    below any usual function calling stack adjustments.  */
(currently_stepping (ecs)
  && prev_pc != stop_pc - DECR_PC_AFTER_BREAK
  && !(step_range_end && INNER_THAN (read_sp (),(step_sp - 16)))));

would be changed to read:

(trap_was_a_software_singlestep
  || ....)

Hmm, looking at the above, on an architecture like the i386, the test 
``prev_pc != stop_pc - DECR_PC_AFTER_BREAK'' is probably false when if 
the code has just stepped off a single byte instruction :-(

enjoy,
Andrew