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

This is a port of the Scalar optimized variant of strspn for amd64
to aarch64.

It utilizes a LUT to speed up the function, a SIMD variant is still
under development.

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/D46396