target/ppc: convert to TranslatorOps

A few changes worth noting:

- Didn't migrate ctx->exception to DISAS_* since the exception field is
  in many cases architecturally relevant.

- Moved the cross-page check from the end of translate_insn to tb_start.

- Removed the exit(1) after a TCG temp leak; changed the fprintf there to
  qemu_log.

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>