lib/libc/aarch64/string: add timingsafe_memcmp() assembly implementation

A port of the amd64 implementation with some slight changes due to
differences in instructions provided by aarch64.

No ASIMD for the same reason as the amd64 code: it's just not particularly
suitable for this application.

Event: EuroBSDcon 2024
Approved by: security (cperciva)
Reviewed by: getz, cperciva
Differential Revision: https://reviews.freebsd.org/D46758