crypto/aes: rewrite ppc64le block crypting

crypto/aes: rewrite ppc64le block crypting

This unrolls the counter loop and simplifies the load/storing
of text/ciphertext and keys by using unaligned VSX memory
operations.

Performance delta on POWER9:

name     old time/op   new time/op    delta
Encrypt   19.9ns ± 0%    14.9ns ± 0%  -24.95%
Decrypt   19.8ns ± 0%    14.6ns ± 0%  -26.12%

Change-Id: Iba98d5c1d88c6bead45bc04c97ae64bcb6fc9f21
Reviewed-on: https://go-review.googlesource.com/c/go/+/404354
Run-TryBot: Paul Murphy <murp@ibm.com>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Filippo Valsorda <valsorda@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>