memmem: add a note about other systems which have memmem

memmem started as a GNU extension but is now widely available.

Reviewed by: mhorne (slightly earlier version)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39337

(cherry picked from commit 30da840c627c80800c06edef346d907d6e5066a6)