Add a warning to stub methods in GHC if they are called

[mpickering]

The user application needs to be compiled with threaded for the debugger to work. Currently if you don't compile with threaded then nothing happens without a warning. This was a bit confusing when it happened to me so we should make it an error to call these methods if you haven't enabled -threaded.

[obreitwi]

https://gitlab.haskell.org/ghc/ghc/merge_requests/1673/