EXTRA_OBJS is not the right thing to be adding into the objects to be
put into a SplitObjs library - it causes certain objects to turn up
twice in the resulting library (eg. if EXTRA_OBJS duplicates some
objects also found in HS_OBJS).

In fact, EXTRA_OBJS is generally bogus and now isn't used anywhere, so
I've removed it.

Should fix the problem recently reported with PrimopWrappers occurring
twice in the libHSbase.a archive.