Don't handle unavailable/optimized-out in spu_software_single_step

Don't handle unavailable/optimized-out in spu_software_single_step

When we do software single step, frame is always the innermost one,
so it is impossible to get unavailable/optimized-out errors.

gdb:

2016-11-11  Yao Qi  <yao.qi@linaro.org>

	* spu-tdep.c (spu_software_single_step): Don't call
	get_frame_register_bytes, call get_frame_register_unsigned
	instead.