As of today, the x86_64-gnu port of glibc is fully and officially upstream! 🎉
You can build it by fetching the latest Git master and running
$ ../configure --host=x86_64-gnu && make
(If you want to actually *run* it on today's GNU Mach, you'll need to apply a little patch: change BRK_START in x86_64/vm_param.h to 0x20000000)
Next up: seeing what it would take to build and run the Hurd itself! I already have the core libraries and servers building.