crypto/sha256: improve performance of loong64

1. Replaced WORD with instruction REVB2W.
2. Simplified the implementation of Ch and Maj by reducing instructions, refer to the implementation of riscv64.

goos: linux
goarch: loong64
pkg: crypto/sha256
cpu: Loongson-3A6000-HV @ 2500.00MHz
                  |  bench.old  |             bench.new              |
                  |   sec/op    |   sec/op     vs base               |
Hash8Bytes/New      313.9n ± 0%   293.4n ± 0%  -6.53% (p=0.000 n=10)
Hash8Bytes/Sum224   324.0n ± 0%   304.2n ± 0%  -6.11% (p=0.000 n=10)
Hash8Bytes/Sum256   322.8n ± 0%   301.8n ± 0%  -6.51% (p=0.000 n=10)
Hash1K/New          4.513µ ± 0%   4.183µ ± 0%  -7.31% (p=0.000 n=10)
Hash1K/Sum224       4.522µ ± 0%   4.189µ ± 0%  -7.36% (p=0.000 n=10)
Hash1K/Sum256       4.522µ ± 0%   4.190µ ± 0%  -7.34% (p=0.000 n=10)
Hash8K/New          33.92µ ± 0%   31.42µ ± 0%  -7.38% (p=0.000 n=10)
Hash8K/Sum224       33.94µ ± 0%   31.42µ ± 0%  -7.40% (p=0.000 n=10)
Hash8K/Sum256       33.94µ ± 0%   31.42µ ± 0%  -7.41% (p=0.000 n=10)
geomean             3.662µ        3.404µ       -7.04%

goos: linux
goarch: loong64
pkg: crypto/sha256
cpu: Loongson-3A5000 @ 2500.00MHz
                  |  bench.old  |             bench.new              |
                  |   sec/op    |   sec/op     vs base               |
Hash8Bytes/New      382.2n ± 0%   357.3n ± 0%  -6.51% (p=0.000 n=10)
Hash8Bytes/Sum224   392.3n ± 0%   367.0n ± 0%  -6.45% (p=0.000 n=10)
Hash8Bytes/Sum256   393.9n ± 0%   368.8n ± 0%  -6.37% (p=0.000 n=10)
Hash1K/New          5.173µ ± 0%   4.725µ ± 0%  -8.66% (p=0.000 n=10)
Hash1K/Sum224       5.189µ ± 0%   4.742µ ± 0%  -8.62% (p=0.000 n=10)
Hash1K/Sum256       5.188µ ± 0%   4.742µ ± 0%  -8.60% (p=0.000 n=10)
Hash8K/New          38.75µ ± 0%   35.34µ ± 0%  -8.78% (p=0.000 n=10)
Hash8K/Sum224       38.77µ ± 0%   35.35µ ± 0%  -8.80% (p=0.000 n=10)
Hash8K/Sum256       38.76µ ± 0%   35.35µ ± 0%  -8.80% (p=0.000 n=10)
geomean             4.277µ        3.936µ       -7.96%

Change-Id: I561f6db118d05fe44485af8ea25df85afa6905a3
Reviewed-on: https://go-review.googlesource.com/c/go/+/668775


Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>