ia64: switch handling of 'foreign import wrapper' (FIW) to libffi

I tried to build darcs-2.4.4 with ghc-6.12.3 and got coredumps when darcs is used
in interactive mode. I tried test from ticket #3516 and found out FIW code is broken.
Instead of fixing it I just switched to libffi. Result built successfully, passed
'foreign import wrapper' test from ticket #3516 and builds working darcs.