fix runghc's GHC detection logic to cover the "in-tree Hadrian build" scenario

Before this patch, runghc would only run the GHC detection logic on Windows and
assume that it was invoked through a wrapper script on all other platforms.
This patch lifts this limitation and makes that logic work for the scenario
where someone is calling the runghc executable directly, without passing an
explicit path to GHC.

utils/runghc/Main.hs

@@ -65,6 +65,11 @@ main = do
 -- live, we check for the existence of ghc. If we can't find it, we assume that
 -- we're building ghc from source, in which case we fall back on ghc-stage2.
 -- (See #1185.)
+--
+-- In-tree Hadrian builds of GHC also happen to give us a wrapper-script-less
+-- runghc. In those cases, 'getExecPath' returns the directory where runghc
+-- lives, which is also where the 'ghc' executable lives, so the guessing logic
+-- covers this scenario just as nicely.
 findGhc :: FilePath -> IO FilePath
 findGhc path = do
     let ghcDir = takeDirectory (normalise path)
@@ -207,5 +212,5 @@ getExecPath = try_size 2048 -- plenty, PATH_MAX is 512 under Win32.
 foreign import WINDOWS_CCONV unsafe "windows.h GetModuleFileNameW"
   c_GetModuleFileName :: Ptr () -> CWString -> Word32 -> IO Word32
 #else
-getExecPath = return Nothing
+getExecPath = Just <$> getExecutablePath
 #endif