s390x assembly pack: implement OPENSSL_rdtsc as STCKF

.. if avalable. STCK has an artificial delay to ensure uniqueness
which can result in a performance penalty if used heavily
concurrently.

Signed-off-by: Patrick Steuer <patrick.steuer@de.ibm.com>

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5284)