target/s390x: cleanup cpu.h

Let's reshuffle the function prototypes so we get a cleaner outline
of the files.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20170818114353.13455-19-david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>