s390x assembly pack: implement OPENSSL_rdtsc as STCKF
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 <015edf36635e030d5e009e08808a59984e629740@openssl.org>
Reviewed-by: Rich Salz <c04971a99e5a9ee80eaab4b1deb37e845b0bd697@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5284)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl