patch applied (ghc): Use a primop for getting the fields of the AP_STACK rather than an FFI call

[Simon Marlow]

Thu May  3 08:19:41 PDT 2007  Simon Marlow <simonmar at microsoft.com>
  * Use a primop for getting the fields of the AP_STACK rather than an FFI call
  This means we can avoid some StablePtrs, and also catch cases where
  the AP_STACK has been evaluated (this can happen with :history, see
  the hist001 test).

    M ./compiler/main/InteractiveEval.hs -17 +24
    M ./compiler/prelude/primops.txt.pp +4
    M ./rts/Interpreter.c -19
    M ./rts/PrimOps.cmm -5 +10