lib/libc/aarch64/string: add strlen SIMD implementation

Adds a SIMD enhanced strlen for Aarch64. It takes inspiration from
the amd64 implementation but I struggled getting the performance I
had hoped for on cores like the Graviton3 when compared to the
existing implementation from Arm Optimized Routines.

See the DR for bechmark results.

Tested by: fuz (exprun)
Reviewed by: fuz, emaste
Sponsored by: Google LLC (GSoC 2024)
PR: 281175
Differential Revision: https://reviews.freebsd.org/D45623