target/s390x: Make MC raise specification exception when class >= 16

target/s390x: Make MC raise specification exception when class >= 16

MC requires bit positions 8-11 (upper 4 bits of class) to be zeros,
otherwise it must raise a specification exception.

Cc: qemu-stable@nongnu.org
Fixes: 20d143e2cab8 ("s390x/tcg: Implement MONITOR CALL")
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Message-Id: <20230724082032.66864-6-iii@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
(cherry picked from commit 9c028c057adce49304c6e4a51f6b426bd4f8f6b8)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>