pkgs/top-level/release-haskell.nix: re-structure Hydra job set

The new job set has the following structure:

  pkg.ghc762.x86_64-linux  = pkgs_x86_64_linux.haskellPackages_ghc762.pkg;
  pkg.ghc762.i686-linux    = pkgs_i686_linux.haskellPackages_ghc762.pkg;
  pkg.ghc6123.x86_64-linux = pkgs_x86_64_linux.haskellPackages_ghc6123.pkg;
  pkg.ghc6123.i686-linux   = pkgs_i686_linux.haskellPackages_ghc6123.pkg;

This gives us (in theory) the ability to generate a Hydra page that displays
the build status of a package across all versions of GHC and all systems. Right
now, Hydra is not up to it, but Eelco says the feature is "on the todo list".

This file doesn't specify the supported build systems explicitly. Instead, that
information is taken from the respective pkg.meta.platforms attribute.