math/big: implement addMulVVW in riscv64 assembly

This provides an assembly implementation of addMulVVW for riscv64,
processing up to four words per loop, resulting in a significant
performance gain.

On a StarFive VisionFive 2:

                   │ addmulvvw.1  │             addmulvvw.2             │
                   │    sec/op    │   sec/op     vs base                │
AddMulVVW/1-4         65.49n ± 0%   50.79n ± 0%  -22.44% (p=0.000 n=10)
AddMulVVW/2-4         82.81n ± 0%   66.83n ± 0%  -19.29% (p=0.000 n=10)
AddMulVVW/3-4        100.20n ± 0%   82.87n ± 0%  -17.30% (p=0.000 n=10)
AddMulVVW/4-4        117.50n ± 0%   84.20n ± 0%  -28.34% (p=0.000 n=10)
AddMulVVW/5-4         134.9n ± 0%   100.3n ± 0%  -25.69% (p=0.000 n=10)
AddMulVVW/10-4        221.7n ± 0%   164.4n ± 0%  -25.85% (p=0.000 n=10)
AddMulVVW/100-4       1.794µ ± 0%   1.250µ ± 0%  -30.32% (p=0.000 n=10)
AddMulVVW/1000-4      17.42µ ± 0%   12.08µ ± 0%  -30.68% (p=0.000 n=10)
AddMulVVW/10000-4     254.9µ ± 0%   214.8µ ± 0%  -15.75% (p=0.000 n=10)
AddMulVVW/100000-4    2.569m ± 0%   2.178m ± 0%  -15.20% (p=0.000 n=10)
geomean               1.443µ        1.107µ       -23.29%

                   │ addmulvvw.1  │              addmulvvw.2              │
                   │     B/s      │      B/s       vs base                │
AddMulVVW/1-4        932.0Mi ± 0%   1201.6Mi ± 0%  +28.93% (p=0.000 n=10)
AddMulVVW/2-4        1.440Gi ± 0%    1.784Gi ± 0%  +23.90% (p=0.000 n=10)
AddMulVVW/3-4        1.785Gi ± 0%    2.158Gi ± 0%  +20.87% (p=0.000 n=10)
AddMulVVW/4-4        2.029Gi ± 0%    2.832Gi ± 0%  +39.59% (p=0.000 n=10)
AddMulVVW/5-4        2.209Gi ± 0%    2.973Gi ± 0%  +34.55% (p=0.000 n=10)
AddMulVVW/10-4       2.689Gi ± 0%    3.626Gi ± 0%  +34.86% (p=0.000 n=10)
AddMulVVW/100-4      3.323Gi ± 0%    4.770Gi ± 0%  +43.54% (p=0.000 n=10)
AddMulVVW/1000-4     3.421Gi ± 0%    4.936Gi ± 0%  +44.27% (p=0.000 n=10)
AddMulVVW/10000-4    2.338Gi ± 0%    2.776Gi ± 0%  +18.69% (p=0.000 n=10)
AddMulVVW/100000-4   2.320Gi ± 0%    2.736Gi ± 0%  +17.93% (p=0.000 n=10)
geomean              2.109Gi         2.749Gi       +30.36%

Change-Id: I6c7ee48233c53ff9b6a5a9002675886cd9bff5af
Reviewed-on: https://go-review.googlesource.com/c/go/+/595400


Reviewed-by: Meng Zhuo <mengzhuo1203@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Mark Ryan <markdryan@rivosinc.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>