crypto/sha256: Use bits.RotateLeft32 instead of ad-hoc implementation

Improves readability of the generic implementation.

Updates #31456.

Benchmarks (i7-4980HQ CPU)

name          old time/op    new time/op    delta
Hash8Bytes-8     339ns ± 3%     337ns ± 2%   ~     (p=0.595 n=5+5)
Hash1K-8        5.12µs ± 6%    4.97µs ± 6%   ~     (p=0.310 n=5+5)
Hash8K-8        37.6µs ± 5%    38.1µs ± 6%   ~     (p=0.841 n=5+5)

name          old speed      new speed      delta
Hash8Bytes-8  23.6MB/s ± 3%  23.8MB/s ± 3%   ~     (p=0.690 n=5+5)
Hash1K-8       200MB/s ± 6%   206MB/s ± 5%   ~     (p=0.310 n=5+5)
Hash8K-8       218MB/s ± 5%   215MB/s ± 6%   ~     (p=0.841 n=5+5)

Change-Id: Ic488841699138efde76e900bce1dd38fdbc88ec6
Reviewed-on: https://go-review.googlesource.com/c/go/+/171731


Reviewed-by: Ilya Tokar <tocarip@gmail.com>
Run-TryBot: Ilya Tokar <tocarip@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>