crypto/internal/fips140/aes/gcm: use aes.EncryptBlockInternal on ppc64x and s390x

crypto/internal/fips140/aes/gcm: use aes.EncryptBlockInternal on ppc64x and s390x

Left them out of CL 636775 because I did a search by reference, which
does not span architectures.

Fixes crypto/cipher.TestFIPSServiceIndicator failure on ppc64x and s390x.

For #69536

Change-Id: I34b49705a7099066e8c3871a7a34b394a9298e98
Reviewed-on: https://go-review.googlesource.com/c/go/+/637175
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>