y2038: hurd: Provide __clock_gettime64 function

For Linux glibc ports the __TIMESIZE == 64 ensures proper aliasing for
__clock_gettime64 (to __clock_gettime).
When __TIMESIZE != 64 (like ARM32, PPC) the glibc expects separate definition
of the __clock_gettime64.

The HURD port only provides __clock_gettime, so this patch adds
__clock_gettime64 as a tiny wrapper on it.
Acked-by: Samuel Thibault <samuel.thibault@ens-lyon.org>