Much like the existing in6addr_any global and the IN6ADDR_ANY_INIT macro, our LibC is also expected to export the in6addr_loopback global and the IN6ADDR_LOOPBACK_INIT constant. These were found by the stress-ng port.
These should allow users to receive the names of network interfaces in the system, but for now these are only stubs required to compile some ports.
This lets us build some of the ports without patching out any of their IPv6 stuff.