crypto/cipher: 8K benchmarks for AES stream modes

Some parallelizable cipher modes may achieve peak performance for larger
block sizes. For this reason the AES-GCM mode already has an 8K
benchmark alongside the 1K version. This change introduces 8K benchmarks
for additional AES stream cipher modes.

Updates #20967

Change-Id: If97c6fbf31222602dcc200f8f418d95908ec1202
Reviewed-on: https://go-review.googlesource.com/136897


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>