target/s390x: Implement STCK et al for CONFIG_USER_ONLY

target/s390x: Implement STCK et al for CONFIG_USER_ONLY

This is a non-privileged instruction that was only implemented
for system mode.  However, the stck instruction is used by glibc,
so this was causing SIGILL for programs run under debian stretch.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20190212053044.29015-3-richard.henderson@linaro.org>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>