Add -stubdir option to control location of generated stub files.  Also
do some clean up while I'm here - remove hscStubCOut/hscStubHOut from
DynFlags, and add

  mkStubPaths :: DynFlags -> Module -> ModLocation -> (FilePath,FilePath)

to Finder.  (this seemed better than caching the stub paths in every
ModLocation, because they are rarely needed and only present in home
modules, and are easily calculated from other available information).

-stubdir behaves in exactly the same way as -odir and -hidir.