Commit 31330e6c ("linux-user/s390x: Implement setup_sigtramp")
removed an unused field from rt_sigframe, disturbing offsets of other
fields and breaking unwinding from signal handlers (e.g. libgcc's
s390_fallback_frame() relies on this struct having a specific layout).
Restore the field and add a comment.

Reported-by: Ulrich Weigand <ulrich.weigand@de.ibm.com>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Fixes: 31330e6c ("linux-user/s390x: Implement setup_sigtramp")
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220503225157.1696774-2-iii@linux.ibm.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>