top-level: Lay the groundwork for {build,host,target}Platform

The long term goal is a big replace:
  { inherit system platform; } => buildPlatform
  crossSystem => hostPlatform
  stdenv.cross => targetPlatform
And additionally making sure each is defined even when not cross compiling.

This commit refactors the bootstrapping code along that vision, but leaves
the old identifiers with their null semantics in place so packages can be
modernized incrementally.

pkgs/top-level/default.nix

@@ -83,7 +83,8 @@ in let
   boot = import ../stdenv/booter.nix { inherit lib allPackages; };
 
   stages = stdenvStages {
-    inherit lib system platform crossSystem config overlays;
+    localSystem = { inherit system platform; };
+    inherit lib crossSystem config overlays;
   };
 
   pkgs = boot stages;