hash/adler32: Unroll loop for extra performance.

hash/adler32: Unroll loop for extra performance.

name         old time/op    new time/op    delta
Adler32KB-4     592ns ± 0%     447ns ± 0%  -24.49%  (p=0.000 n=19+20)

name         old speed      new speed      delta
Adler32KB-4  1.73GB/s ± 0%  2.29GB/s ± 0%  +32.41%  (p=0.000 n=20+20)

Change-Id: I38990aa66ca4452a886200018a57c0bc3af30717
Reviewed-on: https://go-review.googlesource.com/21880
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>