s390x assembly pack: add OPENSSL_s390xcap environment variable.

The OPENSSL_s390xcap environment variable is used to set bits in the s390x
capability vector to zero. This simplifies testing of different code paths.

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

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6813)