sha/asm/keccak1600-s390x.pl: resolve -march=z900 portability issue.

Negative displacement in memory references was not originally specified,
so that for maximum coverage one should abstain from it, just like with
any other extension. [Unless it's guarded by run-time switch, but there
is no switch in keccak1600-s390x.]

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7239)