internal/chacha8rand: implement func block in assembly

internal/chacha8rand: implement func block in assembly

Benchmark result on Loongson-3A6000:
goos: linux
goarch: loong64
pkg: math/rand/v2 + internal/chacha8rand
cpu: Loongson-3A6000-HV @ 2500.00MHz
                         |  bench.old   |              bench.new              |
                         |    sec/op    |   sec/op     vs base                |
ChaCha8MarshalBinary        67.39n ± 0%   65.96n ± 0%   -2.12% (p=0.000 n=10)
ChaCha8MarshalBinaryRead    80.93n ± 0%   78.31n ± 0%   -3.23% (p=0.000 n=10)
ChaCha8                    10.610n ± 0%   5.129n ± 0%  -51.66% (p=0.000 n=10)
ChaCha8Read                 51.30n ± 0%   28.05n ± 0%  -45.31% (p=0.000 n=10)
Block                      218.50n ± 0%   45.48n ± 0%  -79.19% (p=0.000 n=10)
geomean                     57.86n        32.05n       -44.62%

Benchmark result on Loongson-3A5000:
goos: linux
goarch: loong64
pkg: math/rand/v2 + internal/chacha8rand
cpu: Loongson-3A5000 @ 2500.00MHz
                         |  bench.old   |              bench.new              |
                         |    sec/op    |   sec/op     vs base                |
ChaCha8MarshalBinary        116.3n ± 0%   116.6n ± 0%   +0.26% (p=0.015 n=10)
ChaCha8MarshalBinaryRead    142.6n ± 0%   142.0n ± 0%   -0.39% (p=0.000 n=10)
ChaCha8                    16.270n ± 0%   6.848n ± 0%  -57.91% (p=0.000 n=10)
ChaCha8Read                 78.65n ± 0%   47.39n ± 1%  -39.74% (p=0.000 n=10)
Block                      301.50n ± 0%   91.85n ± 0%  -69.53% (p=0.000 n=10)
geomean                     91.45n        54.79n       -40.09%

Change-Id: I64d80c81d2df288fecff80ae23ef89f0fb54cdfa
Reviewed-on: https://go-review.googlesource.com/c/go/+/664035
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>