lib/libc/aarch64/string: add optimized strpbrk & strsep implementations

These are direct copies from the amd64 string functions using the
optimized strcspn from D46398

Tested by: fuz (exprun)
Reviewed by: fuz, emaste
Sponsored by: Google LLC (GSoC 2024)
PR: 281175
Differential Revision: https://reviews.freebsd.org/D46399