crypto/aes: add optimized AES-CTR for ppc64le

crypto/aes: add optimized AES-CTR for ppc64le

This should be a relatively trivial implementation for PPC64
with minor optimizations depending on GOPPC64 value.
GOPPC64=power9 showed about 5% improvement on power10.

Performance is substantially improved on power10 (compiled
with GOPPC64=power9)

          │   p10.old    │                p10.new                │
          │     B/s      │      B/s       vs base                │
AESCTR/50   632.9Mi ± 0%   1022.4Mi ± 1%   +61.54% (p=0.002 n=6)
AESCTR/1K   798.8Mi ± 0%   4327.3Mi ± 0%  +441.72% (p=0.002 n=6)
AESCTR/8K   828.8Mi ± 0%   5799.6Mi ± 0%  +599.77% (p=0.002 n=6)

And power8:

          │    p8.old    │                p8.new                 │
          │     B/s      │      B/s       vs base                │
AESCTR/50   291.6Mi ± 0%    452.4Mi ± 0%   +55.17% (p=0.002 n=6)
AESCTR/1K   380.8Mi ± 0%   2291.6Mi ± 0%  +501.71% (p=0.002 n=6)
AESCTR/8K   389.4Mi ± 0%   3028.1Mi ± 0%  +677.56% (p=0.002 n=6)

Change-Id: Icc977e0a844a5b73a47a218f33dbee54b72edf4b
Reviewed-on: https://go-review.googlesource.com/c/go/+/626176
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>
Reviewed-by: Archana Ravindar <aravinda@redhat.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>