s390x: define function to wait for interrupt

Allow the program to wait for an interrupt.

The interrupt handler is in charge to remove the WAIT bit
when it finished handling the interrupt.

Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1594887809-10521-6-git-send-email-pmorel@linux.ibm.com>
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>