On AVR architecture, the compiler apparently sometimes truncates NSEC_PER_TICK
to 16bit value, leading to clock_time2ticks returning incorrect results. This
was encountered while attempting to add tickless OS support for AVR
architecture but seemed to affect non-tickless mode of operation as well.
This patch marks NSEC_PER_USEC (and to be safe, USEC_PER_MSEC and MSEC_PER_SEC
too) as long.