crypto/internal/poly1305: provide optimised assembly for riscv64

[lxq015]

https://go-review.googlesource.com/c/crypto/+/738760

goos: linux
goarch: riscv64
pkg: [golang.org/x/crypto/internal/poly1305](http://golang.org/x/crypto/internal/poly1305)
                   │ sum1305-old  │            sum1305-new              │
                   │    sec/op    │   sec/op     vs base                │
64-4                  195.9n ± 1%   144.8n ± 0%  -26.07% (p=0.000 n=10)
1K-4                  1.853µ ± 0%   1.012µ ± 0%  -45.39% (p=0.000 n=10)
2M-4                  3.627m ± 0%   1.880m ± 0%  -48.17% (p=0.000 n=10)
64Unaligned-4         196.4n ± 1%   174.7n ± 0%  -11.05% (p=0.000 n=10)
1KUnaligned-4         1.853µ ± 0%   1.491µ ± 0%  -19.54% (p=0.000 n=10)
2MUnaligned-4         3.643m ± 0%   2.850m ± 0%  -21.78% (p=0.000 n=10)
Write64-4            126.85n ± 0%   77.40n ± 0%  -38.98% (p=0.000 n=10)
Write1K-4            1784.5n ± 1%   937.2n ± 0%  -47.48% (p=0.000 n=10)
Write2M-4             3.654m ± 2%   1.880m ± 0%  -48.55% (p=0.000 n=10)
Write64Unaligned-4    125.7n ± 0%   107.5n ± 0%  -14.48% (p=0.000 n=10)
Write1KUnaligned-4    1.782µ ± 0%   1.424µ ± 0%  -20.09% (p=0.000 n=10)
Write2MUnaligned-4    3.637m ± 0%   2.850m ± 0%  -21.64% (p=0.000 n=10)
geomean               10.14µ        6.925µ       -31.68%

                   │ sum1305-old  │             sum1305-new-3             │
                   │     B/s      │      B/s       vs base                │
64-4                 311.7Mi ± 1%    421.5Mi ± 0%  +35.25% (p=0.000 n=10)
1K-4                 527.0Mi ± 0%    965.0Mi ± 0%  +83.10% (p=0.000 n=10)
2M-4                 551.4Mi ± 0%   1063.8Mi ± 0%  +92.94% (p=0.000 n=10)
64Unaligned-4        310.8Mi ± 1%    349.3Mi ± 0%  +12.38% (p=0.000 n=10)
1KUnaligned-4        526.9Mi ± 0%    654.9Mi ± 0%  +24.30% (p=0.000 n=10)
2MUnaligned-4        548.9Mi ± 0%    701.8Mi ± 0%  +27.85% (p=0.000 n=10)
Write64-4            481.2Mi ± 0%    788.6Mi ± 0%  +63.89% (p=0.000 n=10)
Write1K-4            547.2Mi ± 1%   1042.0Mi ± 0%  +90.42% (p=0.000 n=10)
Write2M-4            547.4Mi ± 2%   1064.0Mi ± 0%  +94.38% (p=0.000 n=10)
Write64Unaligned-4   485.5Mi ± 0%    567.7Mi ± 0%  +16.94% (p=0.000 n=10)
Write1KUnaligned-4   547.9Mi ± 0%    685.8Mi ± 0%  +25.17% (p=0.000 n=10)
Write2MUnaligned-4   550.0Mi ± 0%    701.9Mi ± 0%  +27.62% (p=0.000 n=10)
geomean              485.5Mi         710.7Mi       +46.37%

Change-Id: I33a3df03382eb42ef062b0366521f02559290482