[S390] Fix checkstack for s390

With -march=z990 and later gcc can use the long displacement facility
insruction lay for stack register handling. This patch adopts checkstack
to catch lay in addition to ahi and aghi.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>