config.mak.uname: set NO_MEMMEM only for functional version

FreeBSD 6 introduced memmem(), but the implementation diverged
from what was standard everywhere else (including our "compat"
fallback).

FreeBSD 10.4 (went EOL in 2018) corrected the functionality bugs
but kept a suboptimal implementation until FreeBSD 11.4 (the last
version of FreeBSD 11, that went EOL in September 2021).

Let's draw the line to require FreeBSD 12 or newer, which allows us
to drop the special casing of FreeBSD 4.x and rely on the platform
implementation of memmem() unconditionally for all versions that are
still being supported.

Suggested-by: Brad Smith <brad@comstyle.com>
Helped-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>