um: time: Use timespec64 for persistent clock

This read_persistent_clock() implementation is the only remaining
caller of set_normalized_timespec(). Using read_persistent_clock64()
and set_normalized_timespec64() instead lets us remove the deprecated
interface in the future and helps make 32-bit arch/um get closer to
working beyond 2038.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Richard Weinberger <richard@nod.at>