Fix cross installation

See https://gitlab.haskell.org/ghc/ghc/-/issues/14297

lib/GHCup.hs

@@ -1618,11 +1618,11 @@ compileGHC targetGhc ov bstrap jobs mbuildConfig patchdir aargs
       Right g    -> pure $ Right g
       Left  bver -> pure $ Left ("ghc-" <> (T.unpack . prettyVer $ bver) <> exeExt)
 
-    (bindist, bmk) <- liftE $ runBuildAction
+    (mBindist, bmk) <- liftE $ runBuildAction
       tmpUnpack
       Nothing
       (do
-        b <- compileBindist bghc tver workdir
+        b <- compileBindist bghc tver workdir ghcdir
         bmk <- liftIO $ B.readFile (build_mk workdir)
         pure (b, bmk)
       )
@@ -1630,6 +1630,8 @@ compileGHC targetGhc ov bstrap jobs mbuildConfig patchdir aargs
     when alreadyInstalled $ do
       lift $ $(logInfo) [i|Deleting existing installation|]
       liftE $ rmGHCVer tver
+
+    forM_ mBindist $ \bindist -> do
       liftE $ installPackedGHC bindist
                                (Just $ RegexDir "ghc-.*")
                                ghcdir
@@ -1670,11 +1672,12 @@ HADDOCK_DOCS = YES|]
                  => Either FilePath FilePath
                  -> GHCTargetVersion
                  -> FilePath
+                 -> FilePath
                  -> Excepts
                       '[FileDoesNotExistError, InvalidBuildConfig, PatchFailed, ProcessError, NotFoundInPATH, CopyError]
                       m
-                      FilePath  -- ^ output path of bindist
+                      (Maybe FilePath)  -- ^ output path of bindist, None for cross
-  compileBindist bghc tver workdir = do
+  compileBindist bghc tver workdir ghcdir = do
     lift $ $(logInfo) [i|configuring build|]
     liftE checkBuildConfig
 
@@ -1695,6 +1698,7 @@ HADDOCK_DOCS = YES|]
             ("./configure" :  maybe mempty
                       (\x -> ["--target=" <> T.unpack x])
                       (_tvTarget tver)
+            ++ ["--prefix=" <> ghcdir]
 #if defined(IS_WINDOWS)
             ++ ["--enable-tarballs-autodownload"]
 #endif
@@ -1711,6 +1715,7 @@ HADDOCK_DOCS = YES|]
           ++ maybe mempty
                    (\x -> ["--target=" <> T.unpack x])
                    (_tvTarget tver)
+          ++ ["--prefix=" <> ghcdir]
 #if defined(IS_WINDOWS)
           ++ ["--enable-tarballs-autodownload"]
 #endif
@@ -1731,6 +1736,11 @@ HADDOCK_DOCS = YES|]
     lift $ $(logInfo) [i|Building (this may take a while)...|]
     lEM $ make (maybe [] (\j -> ["-j" <> fS (show j)]) jobs) (Just workdir)
 
+    if | isCross tver -> do
+          lift $ $(logInfo) [i|Installing cross toolchain...|]
+          lEM $ make ["install"] (Just workdir)
+          pure Nothing
+       | otherwise -> do
           lift $ $(logInfo) [i|Creating bindist...|]
           lEM $ make ["binary-dist"] (Just workdir)
           [tar] <- liftIO $ findFiles
@@ -1754,7 +1764,7 @@ HADDOCK_DOCS = YES|]
           handleIO (throwE . CopyError . show) $ liftIO $ copyFile (workdir </> tar)
                                                                    tarPath
           lift $ $(logInfo) [i|Copied bindist to #{tarPath}|]
-    pure tarPath
+          pure $ Just tarPath
 
   build_mk workdir = workdir </> "mk" </> "build.mk"
 
@@ -1781,6 +1791,9 @@ HADDOCK_DOCS = YES|]
         )
       _ -> pure ()
 
+  isCross :: GHCTargetVersion -> Bool
+  isCross = isJust . _tvTarget
+