providers: Set the size of EC signature on s390.

The s390x provides its custom implementation for the creation of the
ed448 and ed25519 signatures. Unfortunately it does not set the size.
Users that rely of this return parameter end up with wrong values and
will compare wrong sizes of signature.

Set the proper size of the returned signature on success. Set an error
if the signing operation fails.

Fixes: #18912
Signed-off-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Todd Short <todd.short@me.com>
(Merged from https://github.com/openssl/openssl/pull/18928)

(cherry picked from commit bbedc052)