This adds an asm implementation for aes-gcm on ppc64le to improve
performance.

Results on power8:

name                     old time/op    new time/op     delta
AESGCMSeal1K-192           13.4µs ± 0%      3.7µs ± 0%    -72.48%  (p=1.000 n=1+1)
AESGCMOpen1K-192           10.6µs ± 0%      2.9µs ± 0%    -72.97%  (p=1.000 n=1+1)
AESGCMSign8K-192           60.2µs ± 0%      1.3µs ± 0%    -97.88%  (p=1.000 n=1+1)
AESGCMSeal8K-192           80.5µs ± 0%     22.9µs ± 0%    -71.51%  (p=1.000 n=1+1)
AESGCMOpen8K-192           80.5µs ± 0%     21.5µs ± 0%    -73.27%  (p=1.000 n=1+1)

Change-Id: I026bd4f417095a987eda0f521004af90bc964661
Reviewed-on: https://go-review.googlesource.com/c/go/+/191969
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Munday <mike.munday@ibm.com>