Replaces microkernel global variable _k_sys_clock_tick_count with the
global variable _sys_clock_tick_count. This allows both the microkernel
and the nanokernel to use the same variable to track system clock ticks
instead of using two different ones.
Change-Id: Ia4eebf022f59d130ad1882e0e550016527543a45
Signed-off-by: Peter Mitsis <peter.mitsis@windriver.com>