crypto/cipher: make AES-GCM benchmarks match ChaCha20Poly1305 ones

It's useful to compare TLS AEADs. Here are the numbers on my MacBook
with an Intel(R) Core(TM) i7-7920HQ CPU @ 3.10GHz.

name                            speed
AESGCM/Open-128-64-8             692MB/s ± 2%
AESGCM/Seal-128-64-8             568MB/s ± 1%
AESGCM/Open-128-1350-8          3.96GB/s ± 1%
AESGCM/Seal-128-1350-8          3.17GB/s ± 4%
AESGCM/Open-128-8192-8          5.46GB/s ± 2%
AESGCM/Seal-128-8192-8          4.40GB/s ± 3%

name                            speed
AESGCM/Open-256-64-8             602MB/s ± 2%
AESGCM/Seal-256-64-8             508MB/s ± 1%
AESGCM/Open-256-1350-8          3.06GB/s ± 1%
AESGCM/Seal-256-1350-8          2.65GB/s ± 2%
AESGCM/Open-256-8192-8          4.02GB/s ± 3%
AESGCM/Seal-256-8192-8          3.53GB/s ± 2%

name                            speed
Chacha20Poly1305/Open-64-8       385MB/s ± 3%
Chacha20Poly1305/Seal-64-8       396MB/s ± 3%
Chacha20Poly1305/Open-1350-8    1.67GB/s ± 2%
Chacha20Poly1305/Seal-1350-8    1.62GB/s ± 1%
Chacha20Poly1305/Open-8192-8    2.04GB/s ± 2%
Chacha20Poly1305/Seal-8192-8    2.04GB/s ± 3%

Change-Id: I9373ab85bf132b45b41078205259100fa2d46dda
Reviewed-on: https://go-review.googlesource.com/c/go/+/314610


Trust: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>