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/stdenv/cross/default.nix

@@ -1,10 +1,10 @@
 { lib
-, system, platform, crossSystem, config, overlays
+, localSystem, crossSystem, config, overlays
 }:
 
 let
   bootStages = import ../. {
-    inherit lib system platform overlays;
+    inherit lib localSystem overlays;
     crossSystem = null;
     # Ignore custom stdenvs when cross compiling for compatability
     config = builtins.removeAttrs config [ "replaceStdenv" ];
@@ -14,21 +14,25 @@ in bootStages ++ [
 
   # Build Packages
   (vanillaPackages: {
-    inherit system platform crossSystem config overlays;
+    buildPlatform = localSystem;
+    hostPlatform = localSystem;
+    targetPlatform = crossSystem;
+    inherit config overlays;
     # Should be false, but we're trying to preserve hashes for now
     selfBuild = true;
     # It's OK to change the built-time dependencies
     allowCustomOverrides = true;
     stdenv = vanillaPackages.stdenv // {
-      # Needed elsewhere as a hacky way to pass the target
-      cross = crossSystem;
       overrides = _: _: {};
     };
   })
 
   # Run Packages
   (buildPackages: {
-    inherit system platform crossSystem config overlays;
+    buildPlatform = localSystem;
+    hostPlatform = crossSystem;
+    targetPlatform = crossSystem;
+    inherit config overlays;
     selfBuild = false;
     stdenv = if crossSystem.useiOSCross or false
       then let