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.

this includes the bug fix from b2618b6.

Sponsored by: The FreeBSD Foundation
Reported by: yuri, des
Tested by: des
Approved by: mjg
MFC after: 1 week
MFC to: stable/14
PR: 273652
Differential Revision: https://reviews.freebsd.org/D41598

(cherry picked from commit de12a689fad271f5a2ba7c188b0b5fb5cabf48e7)
(cherry picked from commit b2618b651b28fd29e62a4e285f5be09ea30a85d4)