perl: Enable threading on darwin

Perl on darwin (and any other sane platform) has a pretty good threading
support, enable it.

As it turns out, we were building non-multithreaded perl on all systems,
since glibc was not part of the stdenv anymore:

    nix-repl> pkgs = import <nixpkgs> {}

    nix-repl> pkgs.stdenv ? glibc
    false

meaning that the comments were incorrect. Thus, clear up the confusion
and remove the misleading comments, while enabling multithreading by
default. The builds will fail on unsupported platforms, and in this case
the only place is the bootstrap, where we already force
non-multithreaded perl.

As a consequence of the above, this change will cause the full rebuild
of stdenv on all platforms, including linux.