top-level: Introduce targetPackages and a "double link fold"

Each bootstrapping stage ought to just depend on the previous stage, but
poorly-written compilers break this elegence. This provides an easy-enough
way to depend on the next stage: targetPackages. PLEASE DO NOT USE IT
UNLESS YOU MUST!

I'm hoping someday in a pleasant future I can revert this commit :)

pkgs/top-level/stage.nix

@@ -50,6 +50,14 @@
   # us to avoid expensive splicing.
   buildPackages
 
+, # The package set used in the next stage. If null, `__targetPackages` will be
+  # defined internally as the final produced package set itself, just like with
+  # `buildPackages` and for the same reasons.
+  #
+  # THIS IS A HACK for compilers that don't think critically about cross-
+  # compilation. Please do *not* use unless you really know what you are doing.
+  __targetPackages
+
 , # The standard environment to use for building packages.
   stdenv
 
@@ -88,6 +96,8 @@ let
   stdenvBootstappingAndPlatforms = self: super: {
     buildPackages = (if buildPackages == null then self else buildPackages)
       // { recurseForDerivations = false; };
+    __targetPackages = (if __targetPackages == null then self else __targetPackages)
+      // { recurseForDerivations = false; };
     inherit stdenv
       buildPlatform hostPlatform targetPlatform;
   };