lib/libc/amd64/string: add memchr(3) scalar, baseline implementation

This is conceptually similar to strchr(3), but there are
slight changes to account for the buffer having an explicit
buffer length.

Sponsored by: The FreeBSD Foundation
Approved by: mjg
MFC after: 1 week
MFC to: stable/14
Differential Revision: https://reviews.freebsd.org/D41598