Making hadrian/ghci.sh work executing :main

Since !559 (merged) !555 (merged) !554 (merged) you can load GHC into GHCi like normal and run functions to inspect their result and use the GHCi debugger and so on.

However, if you try to invoke :main on any file then you are greeted with a segfault.

If you try to turn on -fexternal-interpreter you run into errors about missing symbols. The solution here is to add the .o files from c-sources to the hadrian/ghci.sh command line but then you can some confusing errors from the linker.

+    let context = vanillaContext Stage0 compiler
+    cSrcs' <- interpretInContext (vanillaContext Stage0 compiler)
+                                (getContextData cSrcs)
+    cObjs <- mapM (objectPath context) cSrcs'
+    need cObjs
+
     -- Find out the arguments that are needed to load a module into the
     -- session
     arg_list <- interpret fake_target getArgs
-    liftIO $ putStrLn (intercalate " " arg_list)
+    liftIO $ putStrLn (intercalate " " (arg_list ++ cObjs))

Perhaps if -fexternal-interpreter is fixed then the segfaults will also stop happening in the normal version.