This DR adds a scalar and a baseline strchrnul(3) implementation for
amd64. This improves the performance of strchrnul(3), strchr(3), and
index(3). Benchmarks similar to those shown in D40693 show [good
results](https://github.com/clausecker/strperf/commit/830f6f93f6602774ade8325938bc7842982561fc)
(here "pre" refers to the generic C version used previously):
```
os: FreeBSD
arch: amd64
cpu: 11th Gen Intel(R) Core(TM) i7-1165G7 @ 2.80GHz
│ strchrnul_pre.out │ strchrnul_scalar.out │ strchrnul_baseline.out │
│ sec/op │ sec/op vs base │ sec/op vs base │
Short 129.68µ ± 3% 59.91µ ± 1% -53.80% (p=0.000 n=20) 44.37µ ± 1% -65.79% (p=0.000 n=20)
Mid 21.15µ ± 0% 19.30µ ± 0% -8.76% (p=0.000 n=20) 12.30µ ± 0% -41.85% (p=0.000 n=20)
Long 13.772µ ± 0% 11.028µ ± 0% -19.92% (p=0.000 n=20) 3.285µ ± 0% -76.15% (p=0.000 n=20)
geomean 33.55µ 23.36µ -30.37% 12.15µ -63.80%
│ strchrnul_pre.out │ strchrnul_scalar.out │ strchrnul_baseline.out │
│ B/s │ B/s vs base │ B/s vs base │
Short 919.3Mi ± 3% 1989.7Mi ± 1% +116.45% (p=0.000 n=20) 2686.8Mi ± 1% +192.28% (p=0.000 n=20)
Mid 5.505Gi ± 0% 6.033Gi ± 0% +9.60% (p=0.000 n=20) 9.466Gi ± 0% +71.97% (p=0.000 n=20)
Long 8.453Gi ± 0% 10.557Gi ± 0% +24.88% (p=0.000 n=20) 35.441Gi ± 0% +319.26% (p=0.000 n=20)
geomean 3.470Gi 4.983Gi +43.62% 9.584Gi +176.22%
```
The benchmarks always check strings that do not have the character we
are looking for. As the code does not distinguish a NUL match from
a match against the searched character, this should not make a difference
in the performance measured.
Piggybacking on this DR we also remove mentions of x86-64-v3 and v4
versions of strlen(3) which ended up not being committed in D40693.