libc: scalar strnlen() in RISC-V assembly

[strajabot]

Summary

Optimized implementation of strnlen() in RISC-V assembly

Performance was measured using strperf on a HiFive Unmatched (SiFive HF105-001) board.

os: FreeBSD
arch: riscv
        │ strnlen_baseline │           strnlen_scalar            │
        │      sec/op      │   sec/op     vs base                │
Short          787.0µ ± 0%   430.9µ ± 1%  -45.24% (p=0.000 n=20)
Mid            621.6µ ± 0%   195.1µ ± 1%  -68.61% (p=0.000 n=20)
Long           569.4µ ± 1%   100.6µ ± 0%  -82.34% (p=0.000 n=20)
geomean        653.1µ        203.7µ       -68.81%

        │ strnlen_baseline │            strnlen_scalar            │
        │      MiB/s       │    MiB/s     vs base                 │
Short           158.8 ± 0%    290.1 ± 1%   +82.62% (p=0.000 n=20)
Mid             201.1 ± 0%    640.6 ± 1%  +218.59% (p=0.000 n=20)
Long            219.5 ± 1%   1242.9 ± 0%  +466.19% (p=0.000 n=20)
geomean         191.4         613.5       +220.57%

Test Plan

The test suite currently has no test for strnlen. I've tested the function with all different alignments and maxlen up to 128...

All tests that pass before, pass after adding the changes.

[fuz]

Check if you can add a unit test for strnlen. You should be able to adapt the existing memchr unit test for this purpose.

Code looks reasonable.