Support memfd on FreeBSD platform

[heavyrain266]

FreeBSD also have support for memfd, same as linux. They added it years ago for linux compatibility.

See: https://www.freebsd.org/cgi/man.cgi?query=memfd_create&sektion=3&n=1

[i509VCB]

I think this actually needs to be fixed upstream in nix, it only labels android and linux has having memfd

[i509VCB]

Actually there is an issue about it: nix-rust/nix#1775

[i509VCB]

Just need a new nix release at this point.