crypto/aes: fix regression for ppc64x

crypto/aes: fix regression for ppc64x

CL 561080 introduced some failures for CBC and GCM crypto tests that are run during a build resulting in dashboard failures.

Tests that are failing:
- TestCBCEncrypterAES
- TestCBCDecrypterAES
- TestAESGCM
- TestGCMCounterWrap
- TestGCMAsm

This fixes those failures.

Change-Id: I90d6f8d279a3051cf951a1a9628afb8d0b5c2f17
Reviewed-on: https://go-review.googlesource.com/c/go/+/574076
Reviewed-by: Marten Seemann <martenseemann@gmail.com>
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Paul Murphy <murp@ibm.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Eli Bendersky <eliben@google.com>
Auto-Submit: Than McIntosh <thanm@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>