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
(cherry picked from commit 7084133cde6a58412d86bae9f8a55b86141fb304)