crypto/aes: On ppc64le, use better instructions when available

crypto/aes: On ppc64le, use better instructions when available

Several operations emulate instructions available on power9. Use
the GOPPC64_power9 macro provided by the compiler to select the
native instructions if the minimum cpu requirements are met.

Likewise rework the LXSDX_BE to simplify usage when overriding
it. It is only used in one place.

All three configurations are tested via CI.

On POWER9:

pkg:crypto/cipher goos:linux goarch:ppc64le
AESCBCEncrypt1K   949MB/s ± 0%   957MB/s ± 0%  +0.83%
AESCBCDecrypt1K  1.82GB/s ± 0%  1.99GB/s ± 0%  +8.93%
pkg:crypto/aes goos:linux goarch:ppc64le
Encrypt          1.01GB/s ± 0%  1.05GB/s ± 0%  +4.36%
Decrypt           987MB/s ± 0%  1024MB/s ± 0%  +3.77%

Change-Id: I56d0eb845647dd3c43bcad71eb281b499e1d1789
Reviewed-on: https://go-review.googlesource.com/c/go/+/449116
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Auto-Submit: Paul Murphy <murp@ibm.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Joedian Reid <joedian@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Paul Murphy <murp@ibm.com>