crypto/aes: use s390x KMA instruction for AES-GCM if available

crypto/aes: use s390x KMA instruction for AES-GCM if available

Adds support for the cipher message with authentication (KMA)
instruction added in message-security-assist extension 8. This
instruction encapsulates most of the operations required for
AES-GCM and is faster than executing the operations independently.

name          old speed      new speed       delta
AESGCMSeal1K  1.96GB/s ± 0%   6.79GB/s ± 0%  +246.47%  (p=0.000 n=8+10)
AESGCMOpen1K  1.85GB/s ± 0%   5.76GB/s ± 0%  +211.18%  (p=0.000 n=10+10)
AESGCMSign8K  12.0GB/s ± 0%   14.5GB/s ± 0%   +20.43%  (p=0.000 n=10+8)
AESGCMSeal8K  3.75GB/s ± 0%  14.16GB/s ± 0%  +277.57%  (p=0.000 n=9+10)
AESGCMOpen8K  3.70GB/s ± 0%  13.57GB/s ± 0%  +266.50%  (p=0.000 n=10+9)

Change-Id: I57c46573fc5a0bd63c32ce5cba6e37cab85e3de6
Reviewed-on: https://go-review.googlesource.com/73550
Run-TryBot: Michael Munday <mike.munday@ibm.com>
Reviewed-by: Bill O'Farrell <billotosyr@gmail.com>
Reviewed-by: Volodymyr Paprotski <paprots@gmail.com>
Reviewed-by: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>