alpha: switch handling of 'foreign import wrapper' (FIW) to libffi
I tried to build ghc-6.12.3 and found out FIW part of code
does not compile anymore. It uses absent functions under #ifdef.
Instead of fixing it I just switched to libffi. Result built successfully
and passed 'foreign import wrapper' test I wrote for trac ticket Trac #3516.
I didn't try to build -HEAD yet, but this patch only removes code, so
it should not make -HEAD worse.