haskell.compiler.ghc921: check if ld.gold is available in useLdGold
haskell.compiler.ghc921: check if ld.gold is available in useLdGold

Since 4c75874560d114032e3d3ee0e6ffe9dd880a3859 it is possible to
introspect if ld.gold is contained in the used bintools, so we can also
check if it is available before deciding to use it as done in the other
GHC derivations in 0908812372ad7f983645530de1d32e1563adacd3.

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