[spu] Fix single-stepping regression

[spu] Fix single-stepping regression

Switching spu_software_single_step to use a regcache instead of a frame:
https://sourceware.org/ml/gdb-patches/2016-11/msg00355.html
cause a serious regression to SPU single-stepping.

There were two separate problems:
- SPU_LSLR_REGNUM is a pseudo register, so we must use the "cooked"
  regcache methods instead of the "raw" ones to access it.
- When accessing a branch target register, we must only use the first
  four bytes of the 16-byte vector register.  This was done automatically
  by the frame routines, but not by the regcache routines.

gdb/ChangeLog:
2017-11-26  Ulrich Weigand  <uweigand@de.ibm.com>

	* spu-tdep.c (spu_software_single_step): Access SPU_LSLR_REGNUM as
	"cooked" register.  Access only first four bytes of branch target
	registers.