crypto/internal/fips140/sha3: remove usages of WORD for s390x

crypto/internal/fips140/sha3: remove usages of WORD for s390x

We support KIMD and KLMD now, paves the way for banning usage of BYTE
and WORD instructions in crypto assembly.

Change-Id: I0f93744663f23866b2269591db70389e0c77fa4a
Reviewed-on: https://go-review.googlesource.com/c/go/+/671095
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>