Reorder assignments in cmn_systimer_get() to avoid race.

Before, it was possible for the system timer counter to wrap from high to low
after its value had been read but before the value of cmn_systimer_counter
was read and so the resulting sum of those values would be too large.

By reordering the assignments, we can now detect if cmn_systimer_counter has
been advanced while reading the system timer counter value and, if so, repeat
the assignments to ensure consistent values.