target/openrisc: convert to TranslatorOps

target/openrisc: convert to TranslatorOps

Notes:

- Changed the num_insns test in insn_start to check for
  dc->base.num_insns > 1, since when tb_start is first
  called in a TB, base.num_insns is already set to 1.

- Removed DISAS_NEXT from the switch in tb_stop; use
  DISAS_TOO_MANY instead.

- Added an assert_not_reached on tb_stop for DISAS_NEXT
  and the default case.

- Merged the two separate log_target_disas calls into the
  disas_log op.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Cc: Stafford Horne <shorne@gmail.com>
Signed-off-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>