Reapply patch from r62553, with a fix to avoid looking for an ffi.h that isn't

there.

This changes the interpreter to use libffi. After this patch, the interpreter
will barely be able to call any external functions if built on a system without
libffi installed (just enough to pass 'make check' really). But with libffi,
we can now call any function that isn't variadic or taking a struct or vector
parameter (but pointer to struct is fine). Patch by Alexei Svitkine!

llvm-svn: 63723