s390x: Fix gcc 12 warning about array bounds

gcc 12 warns about pointer constant <4k dereference.
Silence the warning by using the extern lowcore symbol to derive the
pointers. This way gcc cannot conclude that the pointer is <4k.

Signed-off-by: Janis Schoetterl-Glausch <scgl@linux.ibm.com>
Message-Id: <20220608122953.1051952-3-scgl@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>