time: Introduce glibc's internal struct __itimerspec64

This change provides the glibc's internal struct itimerspec representation,
which is explicitly supporting 64 bit time (by using struct __timespec64).

Such representation is necessary to provide correct time after Y2038
(time_t overflow) on devices with __TIMESIZE == 32.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>