Optimize with ARMv8 SHA1 instructions.
Results (Cortex-A72)

name             old time/op    new time/op     delta
Hash8Bytes-64      1.06µs ± 4%     0.56µs ± 4%   -47.19%  (p=0.008 n=5+5)
Hash320Bytes-64    3.92µs ± 1%     0.82µs ± 2%   -79.07%  (p=0.008 n=5+5)
Hash1K-64          10.2µs ± 2%      1.5µs ± 2%   -85.71%  (p=0.008 n=5+5)
Hash8K-64          73.9µs ± 1%      7.6µs ± 1%   -89.66%  (p=0.008 n=5+5)

name             old speed      new speed       delta
Hash8Bytes-64    7.55MB/s ± 4%  14.29MB/s ± 4%   +89.27%  (p=0.008 n=5+5)
Hash320Bytes-64  81.6MB/s ± 1%  390.0MB/s ± 2%  +377.64%  (p=0.008 n=5+5)
Hash1K-64         100MB/s ± 2%    701MB/s ± 2%  +599.65%  (p=0.008 n=5+5)
Hash8K-64         111MB/s ± 1%   1072MB/s ± 1%  +867.44%  (p=0.008 n=5+5)

Change-Id: I84397f980db9518f4150ac4c5ffa2c5a97a34444
Reviewed-on: https://go-review.googlesource.com/61550Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>