These changes are compatible with clang-format 16 and will be mandatory
when we eventually bump clang-format version. So, since there are no
real downsides, let's commit them now.
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.