Re: MLton broken FFI on AMD64???

* Wesley W. Terpstra:

> ... either we need to add another tag to imports like:
> _import "printf" stdarg : ... ;
> ... or we should just always set rax for every FFI call on AMD64?

The latter is what libffi is doing, FWIW.  Given that varargs are
somewhat complicated and expensive on amd64 anyway, this shouldn't
matter much.