math/big: implement mulAddVWW in riscv64 assembly

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

On a StarFive VisionFive 2:

                   │ muladdvww.1  │             muladdvww.2             │
                   │    sec/op    │   sec/op     vs base                │
MulAddVWW/1-4         68.18n ± 0%   65.49n ± 0%   -3.95% (p=0.000 n=10)
MulAddVWW/2-4         82.81n ± 0%   78.85n ± 0%   -4.78% (p=0.000 n=10)
MulAddVWW/3-4         97.49n ± 0%   72.18n ± 0%  -25.96% (p=0.000 n=10)
MulAddVWW/4-4        112.20n ± 0%   85.54n ± 0%  -23.76% (p=0.000 n=10)
MulAddVWW/5-4        126.90n ± 0%   98.90n ± 0%  -22.06% (p=0.000 n=10)
MulAddVWW/10-4        200.3n ± 0%   144.3n ± 0%  -27.96% (p=0.000 n=10)
MulAddVWW/100-4      1532.0n ± 0%   860.0n ± 0%  -43.86% (p=0.000 n=10)
MulAddVWW/1000-4     14.757µ ± 0%   8.076µ ± 0%  -45.27% (p=0.000 n=10)
MulAddVWW/10000-4     204.0µ ± 0%   137.1µ ± 0%  -32.77% (p=0.000 n=10)
MulAddVWW/100000-4    2.066m ± 0%   1.382m ± 0%  -33.12% (p=0.000 n=10)
geomean               1.311µ        950.0n       -27.51%

                   │ muladdvww.1  │             muladdvww.2              │
                   │     B/s      │     B/s       vs base                │
MulAddVWW/1-4        895.1Mi ± 0%   932.0Mi ± 0%   +4.11% (p=0.000 n=10)
MulAddVWW/2-4        1.440Gi ± 0%   1.512Gi ± 0%   +5.02% (p=0.000 n=10)
MulAddVWW/3-4        1.834Gi ± 0%   2.477Gi ± 0%  +35.07% (p=0.000 n=10)
MulAddVWW/4-4        2.125Gi ± 0%   2.787Gi ± 0%  +31.15% (p=0.000 n=10)
MulAddVWW/5-4        2.349Gi ± 0%   3.013Gi ± 0%  +28.28% (p=0.000 n=10)
MulAddVWW/10-4       2.975Gi ± 0%   4.130Gi ± 0%  +38.79% (p=0.000 n=10)
MulAddVWW/100-4      3.891Gi ± 0%   6.930Gi ± 0%  +78.11% (p=0.000 n=10)
MulAddVWW/1000-4     4.039Gi ± 0%   7.380Gi ± 0%  +82.72% (p=0.000 n=10)
MulAddVWW/10000-4    2.922Gi ± 0%   4.346Gi ± 0%  +48.74% (p=0.000 n=10)
MulAddVWW/100000-4   2.884Gi ± 0%   4.313Gi ± 0%  +49.52% (p=0.000 n=10)
geomean              2.321Gi        3.202Gi       +37.95%

Change-Id: If08191607913ce5c7641f34bae8fa5c9dfb44777
Reviewed-on: https://go-review.googlesource.com/c/go/+/595399


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