Support vCont s and S actions with software single step

Support vCont s and S actions with software single step

GDBserver with software single step should be able to claim supporting
vCont s and S actions, so that GDB knows the remote target can do
single step.  It doesn't matter to GDB that the single step in the
remote target is done via hardware or software.

gdb/gdbserver:

2016-07-21  Yao Qi  <yao.qi@linaro.org>

	* server.c (handle_v_requests): Support s and S actions
	if target_supports_software_single_step return true.