Fix ghci on Windows when GHC is dynamically linked

On Windows, we need to prepend "lib" to Haskell DLLs.
(maybe we should be naming the DLLs without the prefix instead? But this
works for now).

compiler/ghci/Linker.lhs

@@ -1195,7 +1195,7 @@ locateLib dflags is_hs dirs lib
      mk_arch_path     dir = dir </> ("lib" ++ lib <.> "a")
 
      hs_dyn_lib_name = lib ++ "-ghc" ++ cProjectVersion
-     mk_hs_dyn_lib_path dir = dir </> mkSOName platform hs_dyn_lib_name
+     mk_hs_dyn_lib_path dir = dir </> mkHsSOName platform hs_dyn_lib_name
 
      so_name = mkSOName platform lib
      mk_dyn_lib_path dir = dir </> so_name

compiler/main/HscTypes.lhs

@@ -37,7 +37,7 @@ module HscTypes (
 
         PackageInstEnv, PackageRuleBase,
 
-        mkSOName, soExt,
+        mkSOName, mkHsSOName, soExt,
 
         -- * Annotations
         prepareAnnotations,
@@ -1796,6 +1796,9 @@ mkSOName platform root
       OSMinGW32 ->           root  <.> "dll"
       _         -> ("lib" ++ root) <.> "so"
 
+mkHsSOName :: Platform -> FilePath -> FilePath
+mkHsSOName platform root = ("lib" ++ root) <.> soExt platform
+
 soExt :: Platform -> FilePath
 soExt platform
     = case platformOS platform of