Resurrect UseLibFFIForAdjustors from bitrot.

* Pass -Irts/dist/build to the C preprocessor to expose libffi headers (ffi.h and ffitarget.h) to foreign import wrappers during the building process of GHC itself.
* Install libffi headers into $(ghcheaderdir) just like any other C headers. Otherwise an installed GHC can't find them when it wants to compile foreign import wrappers.
* Include libffi headers in the bindist for the same reason.