Power8 inline assembly tweaks

Power8 inline assembly tweaks

Further fixes on top of 9e2f316ed.  Writing some doco for gcc on
inline assembly woke me up to some more errors.

- dgemv_kernel_4x4 asm did not mention *ap as a memory input, and
  *y is both read and write.
- sasum_kernel_32 and casum_kernel_16 did not use %x for a vsx insn
  operand, a problem if the "=f" sum output was ever allocated a vsx
  reg in the altivec set.  This might be possible with inlining and
  future gcc optimisation.