adding isWHNF primop to 5.00.2 native code generator

[Simon Marlow]

>    \begin{code}
>    primCode [res] IsHNF [arg]
>       =3D let res'        =3D amodeToStix res
>             arg'        =3D amodeToStix arg
>             arg_info    =3D StInd PtrRep arg'
>             word_32     =3D StInd WordRep (StIndex PtrRep=20
> arg_info (StInt (-1)))
>             masked_le32 =3D StPrim SrlOp [word_32, StInt 16]
>             masked_be32 =3D StPrim AndOp [word_32, StInt 65535]
>    #ifdef WORDS_BIGENDIAN
>             ty_info     =3D masked_le32
>    #else
>             ty_info     =3D masked_be32
>    #endif
>             not_a_thunk =3D StPrim IntEqOp [ StPrim AndOp=20
> [ty_info, StInt 0x10]
>                                          , StInt 0x0
>                                          ]
>                    -- ToDo: don't hardwire the value of=20
> _THUNK from InfoTables.h
>             assign      =3D StAssign IntRep res' not_a_thunk
>         in
>         returnUs (\ xs -> assign : xs)
>    \end{code}=20

This code looks bogus - it is checking a bit in the info table, which
would have been correct when we used to store flags in the info table,
but that changed a while back.

closure_THUNK() works by getting the type field (i.e.
get_itbl(c)->type), and using that to index into the table of closure
flags (see ghc/rts/ClosureFlags.c), and checking the "THU" flag.  This
is the portable way to do it.  The hacky way to do it would be to
compare against the closure types directly - fortunately you can use the
names rather than the constants, see ghc/compiler/codeGen/SMRep.lhs.

Cheers,
	Simon