lib/libc/amd64/string: add strspn(3) scalar, x86-64-v2 implementation

This is conceptually very similar to the strcspn(3) implementations
from D41557, but we can't do the fast paths the same way.

Sponsored by: The FreeBSD Foundation
Approved by: mjg
MFC after: 1 week
MFC to: stable/14
Differential Revision: https://reviews.freebsd.org/D41567