target/s390x: Do not modify cpu state in s390_cpu_get_psw_mask

target/s390x: Do not modify cpu state in s390_cpu_get_psw_mask

We want to use this function for debugging, and debug should
not modify cpu state (even non-architectural cpu state) lest
we introduce heisenbugs.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Tested-by: jonathan.albrecht <jonathan.albrecht@linux.vnet.ibm.com>
Tested-by: <ruixin.bao@ibm.com>
Message-Id: <20210615030744.1252385-3-richard.henderson@linaro.org>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>