In order to properly present these arguments, we need to add
code to target/s390x to record LowCore parameters for user-only.

But in the meantime, at least zero the missing last_break
argument, and fixup the comment style in the vicinity.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Message-Id: <20210428193408.233706-12-richard.henderson@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>