libc: update strstr implementation to match musl

musl commits:
122d67f846cb0be2c9e1c3880db9eb9545bbe38c
0239cd0681e889a269fb7691f60e81ef8d081e6b
8f5a820d147da36bcdbddd201b35d293699dacd8

Submitted by: David CARLIER <devnexen_gmail.com>
Obtained from: musl
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D19834