top-level: move top-level {build,host,target}Platform to aliases

deprecation was attempted in e51f736076548459f36a1250de4bf6867f880b66

but had to be reverted in 2a6e4ae49a891adc7c0562fda08b17d60beb1b4f

pkgs/top-level/aliases.nix

···
   zyn-fusion = zynaddsubfx; # Added 2022-08-05
 
   inherit (stdenv.hostPlatform) system; # Added 2021-10-22
+  inherit (stdenv) buildPlatform hostPlatform targetPlatform; # Added 2023-01-09
 
   # LLVM packages for (integration) testing that should not be used inside Nixpkgs:
   llvmPackages_git = recurseIntoAttrs (callPackage ../development/compilers/llvm/git {

pkgs/top-level/stage.nix

···
     inherit stdenv;
   };
 
-  # The old identifiers for cross-compiling. These should eventually be removed,
-  # and the packages that rely on them refactored accordingly.
-  platformCompat = self: super: let
-    inherit (super.stdenv) buildPlatform hostPlatform targetPlatform;
-  in {
-    inherit buildPlatform hostPlatform targetPlatform;
-  };
-
   splice = self: super: import ./splice.nix lib self (adjacentPackages != null);
 
   allPackages = self: super:
···
   # previous bootstrapping phases which have already been overlayed.
   toFix = lib.foldl' (lib.flip lib.extends) (self: {}) ([
     stdenvBootstappingAndPlatforms
-    platformCompat
     stdenvAdapters
     trivialBuilders
     splice