Fix the location of _stub.o files when compiling with -odir.  The
comment from the source:

-- The _stub.c file is derived from the haskell source file (but stored
-- in hscStubCOutName in the dflags for some reason, probably historical).
-- Consequently, we derive the _stub.o filename from the haskell object
-- filename.
--
-- This isn't necessarily the same as the object filename we
-- would get if we just compiled the _stub.c file using the pipeline.
-- For example:
--
--    ghc src/A.hs -odir obj
--
-- results in obj/A.o, and src/A_stub.c.  If we compile src/A_stub.c with
-- -odir obj, we would get obj/src/A_stub.o, which is wrong; we want
-- obj/A_stub.o.