crypto/internal/fips/sha3: test alternative s390x implementation

crypto/internal/fips/sha3: test alternative s390x implementation

The amd64 assembly is always-on, so we don't need to test disabling it.

Fixes #36466
For #69536

Change-Id: I2cd4befcde688a1ba202e61c7119e15454ff6854
Reviewed-on: https://go-review.googlesource.com/c/go/+/617535
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: Roland Shoemaker <roland@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>