libc: scalar memset() in RISC-V assembly

Adds scalar implementation of memset for RISC-V
and updates the relevant manpage

os: FreeBSD
arch: riscv

│ ./results/memset/memset_baseline │   ./results/memset/memset_scalar    │
│              sec/op              │   sec/op     vs base                │

40 527.5µ ± 1% 479.4µ ± 1% -9.12% (p=0.000 n=20)
168 254.5µ ± 1% 216.7µ ± 1% -14.86% (p=0.000 n=20)
2k 169.5µ ± 1% 128.4µ ± 0% -24.24% (p=0.000 n=20)
256k 161.2µ ± 1% 118.6µ ± 1% -26.42% (p=0.000 n=20)
16m 56.58m ± 0% 53.91m ± 0% -4.72% (p=0.000 n=20)
geomean 730.2µ 611.2µ -16.29%

│ ./results/memset/memset_baseline │    ./results/memset/memset_scalar     │
│               B/s                │      B/s       vs base                │

40 452.0Mi ± 1% 497.3Mi ± 1% +10.04% (p=0.000 n=20)
168 936.9Mi ± 1% 1100.4Mi ± 1% +17.45% (p=0.000 n=20)
2k 1.373Gi ± 1% 1.813Gi ± 0% +32.00% (p=0.000 n=20)
256k 1.444Gi ± 1% 1.962Gi ± 1% +35.91% (p=0.000 n=20)
16m 269.7Mi ± 0% 283.1Mi ± 0% +4.96% (p=0.000 n=20)
geomean 750.1Mi 896.1Mi +19.47%

MFC after: 1 month
MFC to: stable/15
Approved by: mhorne, markj (mentor)
Reviewed by: fuz
Sponsored by: Google LLC (GSoc 2024)
Differential Revision: https://reviews.freebsd.org/D45730