Even if I do not own any x86_64 computer, after spending a specific amount of time on doing the i386 port, I started to work on an x86_64 port using qemu. After spending almost as much time on just the i386->x86_64 porting of v7libc as the original v7->i386 porting took for the whole project, I gave up.
I think it mostly works already, but there is some problem with signal handling, the code segfaults when the kernel calls the signal handler. After fixing that, there's a minor segfault in scanf and then libc should work. Another big amount of work would be to port the rest of the tools - less tricky, but probably large in volume.
I have much better things to do than porting the code to a platform I don't own. If anyone volunteers for contributing the port, I am all for it and I would provide all support needed.