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

This patch requires D46170 as it depends on strlcpy being labeled
__memccpy.

It's a direct copy from the amd64 string functions.

Tested by: fuz (exprun)
Reviewed by: fuz, emaste
Sponsored by: Google LLC (GSoC 2024)
PR: 281175
Differential Revision: https://reviews.freebsd.org/D46292