CI: Remove -Wno-stringop-overflow for s390x builds

CI: Remove -Wno-stringop-overflow for s390x builds

Since https://github.com/openssl/openssl/commit/9a788281d91f698d6a229d588b9cb36987549669
it should now build warning-free on s390x, so remove the '-Wno-stringop-overflow'
build option for s390x builds.

If newly added code causes -Wstringop-overflow warnings again, it should
be noted in the CI runs and the newly added code should be fixed accordingly.

Signed-off-by: Ingo Franzki <ifranzki@linux.ibm.com>
Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/27803)