Closes NixOS#252.

This uses the `lib.systems.elaborate` function which builds the
complete attrset as preferred by Nixpkgs by using
`lib.systems.elaborate`, rather than building a partial equivalent.

@noneucat originally implemented it the simplest way possible, re-using
parts of the now unneeded custom functions. We can, instead, simply
pass a system name to `elaborate`, which ends up doing the right thing.

Co-authored-by: Andy Chun <andy@lolc.at>
Co-authored-by: Samuel Dionne-Riel <samuel@dionne-riel.com>