lib/libc/aarch64/string: strcat enable use of SIMD

Call into SIMD strlen and stpcpy for an optimized strcat. Port of
D42600 for amd64.

Tested by: fuz (exprun)
Reviewed by: fuz, emaste
Sponsored by: Google LLC (GSoC 2024)
PR: 281175
Differential Revision: https://reviews.freebsd.org/D46417