See the added comment in all-packages.nix for a more detailed
explanation. This makes the top-level GHC different from
haskellPackages.ghc (which is build->host and used for building the
package set), but more consistent with gcc, gnat etc.
Specifically, pkgsCross.${platform}.buildPackages.ghc will now be a
cross-compiler instead of a native build->build compiler.
Since this change has a slight chance of being disruptive, add a note to
the changelog.
