haskell.compiler.ghc9{6,8,10}: update comment about stage selection

The build platform doesn't matter for checking which stage is our final
stage! Stage2 means host and target are sufficiently similar, Stage1
means host and target may differ.

pkgs/development/compilers/ghc/common-hadrian.nix

@@ -124,9 +124,8 @@
     -- no way to set this via the command line
     finalStage :: Stage
     finalStage = ${
-      # Always build the stage 2 compiler if possible. Note we can currently
-      # assume hostPlatform == buildPlatform.
-      # TODO(@sternenseemann): improve this condition when we can cross-compile GHC
+      # Always build the stage 2 compiler if possible.
+      # TODO(@sternensemann): unify condition with make-built GHCs
       if stdenv.hostPlatform.canExecute stdenv.targetPlatform
       then "Stage2" # native compiler or “native” cross e.g. pkgsStatic
       else "Stage1" # cross compiler