haskell.compiler.ghc981: build stage 2 compiler for “native cross”

When we are building compiler for a platform we can execute ourselves,
we can build a proper stage2 compiler which unlocks some features that
are interesting for e.g. pkgsStatic.

The resulting compiler is technically a native compiler that's prefixed.

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

@@ -116,8 +116,11 @@
     -- no way to set this via the command line
     finalStage :: Stage
     finalStage = ${
-      if stdenv.hostPlatform == stdenv.targetPlatform
-      then "Stage2" # native compiler
+      # 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
+      if stdenv.hostPlatform.canExecute stdenv.targetPlatform
+      then "Stage2" # native compiler or “native” cross e.g. pkgsStatic
       else "Stage1" # cross compiler
     }