pkgs/top-level: use lib.systems.equals for crossSystem

Fixes otherwise equivalent systems being treated as different by
packages that compare `stdenv.*Platform`s using `==` operator.

pkgs/test/default.nix

···
 
   config = callPackage ./config.nix { };
 
+  top-level = callPackage ./top-level { };
+
   haskell = callPackage ./haskell { };
 
   hooks = callPackage ./hooks { };

pkgs/test/top-level/default.nix

···
+{ lib, pkgs, ... }:
+let
+  nixpkgsFun = import ../../top-level;
+in
+lib.recurseIntoAttrs {
+  platformEquality =
+    let
+      configsLocal = [
+        # crossSystem is implicitly set to localSystem.
+        {
+          localSystem = { system = "x86_64-linux"; };
+        }
+        {
+          localSystem = { system = "aarch64-linux"; };
+          crossSystem = null;
+        }
+        # Both systems explicitly set to the same string.
+        {
+          localSystem = { system = "x86_64-linux"; };
+          crossSystem = { system = "x86_64-linux"; };
+        }
+        # Vendor and ABI inferred from system double.
+        {
+          localSystem = { system = "aarch64-linux"; };
+          crossSystem = { config = "aarch64-unknown-linux-gnu"; };
+        }
+      ];
+      configsCross = [
+        # GNU is inferred from double, but config explicitly requests musl.
+        {
+          localSystem = { system = "aarch64-linux"; };
+          crossSystem = { config = "aarch64-unknown-linux-musl"; };
+        }
+        # Cross-compile from AArch64 to x86-64.
+        {
+          localSystem = { system = "aarch64-linux"; };
+          crossSystem = { system = "x86_64-unknown-linux-gnu"; };
+        }
+      ];
+
+      pkgsLocal = map nixpkgsFun configsLocal;
+      pkgsCross = map nixpkgsFun configsCross;
+    in
+    assert lib.all (p: p.buildPlatform == p.hostPlatform) pkgsLocal;
+    assert lib.all (p: p.buildPlatform != p.hostPlatform) pkgsCross;
+    pkgs.emptyFile;
+}

pkgs/top-level/default.nix

···
   localSystem = lib.systems.elaborate args.localSystem;
 
   # Condition preserves sharing which in turn affects equality.
+  #
+  # See `lib.systems.equals` documentation for more details.
+  #
+  # Note that it is generally not possible to compare systems as given in
+  # parameters, e.g. if systems are initialized as
+  #
+  #   localSystem = { system = "x86_64-linux"; };
+  #   crossSystem = { config = "x86_64-unknown-linux-gnu"; };
+  #
+  # Both systems are semantically equivalent as the same vendor and ABI are
+  # inferred from the system double in `localSystem`.
   crossSystem =
-    if crossSystem0 == null || crossSystem0 == args.localSystem
+    let system = lib.systems.elaborate crossSystem0; in
+    if crossSystem0 == null || lib.systems.equals system localSystem
     then localSystem
-    else lib.systems.elaborate crossSystem0;
+    else system;
 
   # Allow both:
   # { /* the config */ } and