Simplify the foreign-export stub processing.

  - DynFlags now has fields for the stub.h and stub.c filenames, for
    consistency with the normal hsc output file name.

  - codeOutput puts the stubs into these files rather than dreaming
    up new temporary names for them

  - now we don't have to move the stubs into the right place in
    DriverPipeline.

  - we do however have to inject the correct #includes into the stub.c
    file when it is generated: I'm now injecting the same includes as
    the .hc file gets plus "RtsAPI.h", which is probably more correct
    than the hacky hardcoded "Stg.h" we had before.