hie-core exe: initDynLinker (#2555)

The hie-core tests are flaky in stack. They can fail with the following
error message

```
hie-core: panic! (the 'impossible' happened)\n  (GHC version 8.6.5 for x86_64-unknown-linux):
        Dynamic linker not initialised

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
```

Explicitly initializing the dynamic linker at session startup should
avoid this issue.

exe/Main.hs

@@ -25,6 +25,7 @@ import Development.IDE.Types.Logger
 import qualified Data.Text as T
 import qualified Data.Text.IO as T
 import Language.Haskell.LSP.Messages
+import Linker
 import Development.IDE.LSP.LanguageServer
 import System.Directory.Extra as IO
 import System.Environment
@@ -125,6 +126,9 @@ showEvent lock (EventFileDiagnostics (toNormalizedFilePath -> file) diags) =
 showEvent lock e = withLock lock $ print e
 
 newSession' :: Cradle -> IO HscEnv
-newSession' cradle = getLibdir >>= \libdir -> runGhc (Just libdir) $ do
-    initializeFlagsWithCradle "" cradle
-    getSession
+newSession' cradle = getLibdir >>= \libdir -> do
+    env <- runGhc (Just libdir) $ do
+        initializeFlagsWithCradle "" cradle
+        getSession
+    initDynLinker env
+    pure env