mandoc: move executableCross assert into meta.broken
mandoc: move executableCross assert into meta.broken

For splicing to work, callPackage needs to be able to attach extra
attributes (e. g. nativeDrv) to pkgsHostTarget.mandoc. If we have the
assert like before, pkgsHostTarget.mandoc would fail to evaluate,
preventing buildPackages.mandoc (which works as expected) to be attached
to the former. This affected cross compilation of the netbsd package set
in particular.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs