libc: scalar strchrnul() in RISC-V assembly

[strajabot]

Summary

Scalar implementation of strchrnul() in RISC-V assembly and changes to the
corresponding manpage.

Performance was benchmarked on a HiFive Unmatched (SiFive HF105-001) board
using: https://github.com/clausecker/strperf

os: FreeBSD
arch: riscv
        │ strchrnul_baseline │          strchrnul_scalar           │
        │       sec/op       │   sec/op     vs base                │
Short            680.2µ ± 5%   435.3µ ± 0%  -36.01% (p=0.000 n=20)
Mid              314.7µ ± 3%   221.4µ ± 0%  -29.63% (p=0.000 n=20)
Long             152.3µ ± 0%   138.5µ ± 0%   -9.08% (p=0.000 n=20)
geomean          319.5µ        237.2µ       -25.75%

        │ strchrnul_baseline │          strchrnul_scalar          │
        │       MiB/s        │   MiB/s     vs base                │
Short             183.8 ± 5%   287.2 ± 0%  +56.27% (p=0.000 n=20)
Mid               397.3 ± 3%   564.6 ± 0%  +42.12% (p=0.000 n=20)
Long              820.5 ± 0%   902.5 ± 0%   +9.99% (p=0.000 n=20)
geomean           391.3        527.0       +34.68%

Test Plan

Tested using the FreeBSD test suite

[fuz]

This looks reasonable. Accepted pending final acceptance test.