s390x: Fix GCM setup

Rework of GCM code did not include s390x causing NULL pointer dereferences on
GCM operations other than AES-GCM on platforms that support kma.  Fix this by
a proper setup of the function pointers.

Fixes: 92c9086e ("Use separate function to get GCM functions")

Signed-off-by: Juergen Christ <jchrist@linux.ibm.com>

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/18862)