target/ppc: convert to DisasContextBase

A couple of notes:

- removed ctx->nip in favour of base->pc_next. Yes, it is annoying,
  but didn't want to waste its 4 bytes.

- ctx->singlestep_enabled does a lot more than
  base.singlestep_enabled; this confused me at first.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>