S390X: Fix Ed448ph

S390X: Fix Ed448ph

CPACF does not support pre-hashing.  This was considered correctly for
Ed25519ph, but not for Ed448ph which lead to errors in the test_evp suite
(test vector 20 - pre-hashing without context string).  Fix this by using the
non-accelerated version of Ed448 also if no context string is provided, but
pre-hashing is performed.

Signed-off-by: Juergen Christ <jchrist@linux.ibm.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/20067)