Honour software single step in fallback of displaced stepping

Honour software single step in fallback of displaced stepping

Hi,
When I run gdb.threads/non-stop-fair-events.exp on arm-linux target,
I see the following message in the debugging log,

  displaced: breakpoint is gone: Thread 22518, step(1)^M
  Sending packet: $vCont;s:p57f3.57f6#9d...
                  ^^^^^^^^^
GDB sends vCont;s by mistake, and GDBserver fails on assert.  GDB
doesn't consider software single step in infrun.c:displaced_step_fixup,

	  /* Go back to what we were trying to do.  */
	  step = currently_stepping (tp);

	  if (debug_displaced)
	    fprintf_unfiltered (gdb_stdlog,
				"displaced: breakpoint is gone: %s, step(%d)\n",
				target_pid_to_str (tp->ptid), step);

	  target_resume (ptid, step, GDB_SIGNAL_0);

The patch is to let GDB consider software single step here.  It fixes
fails in gdb.threads/non-stop-fair-events.exp on arm.

gdb:

2015-04-16  Yao Qi  <yao.qi@linaro.org>

	* infrun.c (maybe_software_singlestep): Declare.
	(displaced_step_fixup): Call maybe_software_singlestep.