lib/libc/aarch64/string: add strcspn optimized implementation

This is a port of the Scalar optimized variant of strcspn for amd64
to aarch64 It utilizes a LUT to speed up the function, a SIMD
variant is still under development.

Performance benchmarks are as usual generated by strperf.

See the DR for benchmark results.

Tested by: fuz (exprun)
Reviewed by: fuz, emaste
Sponsored by: Google LLC (GSoC 2024)
PR: 281175
Differential Revision: https://reviews.freebsd.org/D46398