Convert `ghc-bin.cabal` to use others-extensions

This replaces the previous `default-extensions` by per-file declared
`{-# LANGUAGE ... #-}` pragmas.
Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>

ghc/GhciMonad.hs

+{-# LANGUAGE CPP, FlexibleInstances, UnboxedTuples, MagicHash #-}
 {-# OPTIONS_GHC -fno-cse -fno-warn-orphans #-}
 -- -fno-cse is needed for GLOBAL_VAR's to behave properly
 

ghc/InteractiveUI.hs

+{-# LANGUAGE CPP, MagicHash, NondecreasingIndentation, TupleSections #-}
 {-# OPTIONS -fno-cse #-}
 -- -fno-cse is needed for GLOBAL_VAR's to behave properly
 

ghc/Main.hs

+{-# LANGUAGE CPP, NondecreasingIndentation #-}
 {-# OPTIONS -fno-warn-incomplete-patterns -optc-DNON_POSIX_SOURCE #-}
 
 -----------------------------------------------------------------------------

ghc/ghc-bin.cabal.in

@@ -45,11 +45,17 @@ Executable ghc
     if flag(ghci)
         CPP-Options: -DGHCI
         GHC-Options: -fno-warn-name-shadowing
-        Other-Modules: InteractiveUI, GhciMonad, GhciTags
+        Other-Modules:
+            InteractiveUI
+            GhciMonad
+            GhciTags
         Build-Depends: transformers, haskeline
-        Default-Extensions: UnboxedTuples,
-                    FlexibleInstances,
-                    TupleSections,
-                    MagicHash
+        Other-Extensions:
+            FlexibleInstances
+            MagicHash
+            TupleSections
+            UnboxedTuples
 
-    Default-Extensions: CPP, NondecreasingIndentation
+    Other-Extensions:
+        CPP
+        NondecreasingIndentation