(slightly optimistically adding this to the 6.10 milestone).
For Win64 we need:
A working mingw64 port, which doesn't exist yet. There are experimental
patches to gcc and binutils, I believe. Alternatively, we could do
the Windows Native port first, and avoid mingw.
Win64 has a different calling convention, so all the usual stuff needs
to be ported: the native code generator, adjustors, FFI support in GHCi, etc.
Perhaps libffi would make this easier (see #793 (closed)).