lib/libc/amd64/string: implement strpbrk() through strcspn()

This lets us use our optimised strcspn() routine for strpbrk() calls.

Sponsored by: The FreeBSD Foundation
Tested by: developers@, exp-run
Approved by: mjg
MFC after: 1 month
MFC to: stable/14
PR: 275785
Differential Revision: https://reviews.freebsd.org/D41980