target/s390x: convert to DisasContextBase

target/s390x: convert to DisasContextBase

Notes:

- Did not convert {num,max}_insns and is_jmp, since the corresponding
  code will go away in the next patch.

- Avoided a checkpatch error in use_exit_tb.

- As suggested by David, (1) Drop ctx.pc and use
  ctx.base.pc_next instead, and (2) Rename ctx.next_pc to
  ctx.pc_tmp and add a comment about it.

Acked-by: Cornelia Huck <cohuck@redhat.com>
Suggested-by: David Hildenbrand <david@redhat.com>
Reviewed-by:  David Hildenbrand <david@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Cornelia Huck <cohuck@redhat.com>
Cc: Alexander Graf <agraf@suse.de>
Cc: qemu-s390x@nongnu.org
Signed-off-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>