kernel/timeout: Fix synchronization in z_tick_get_32()
kernel/timeout: Fix synchronization in z_tick_get_32()

The previous comment correctly and carefully explained why the 64 bit
value in curr_tick doesn't require locking when reading only the low
32 bits.

It completely missed the fact that the calculation of elapsed time and
the read of curr_tick ABSOLUTELY DO require locking, because the
former is expressed in terms of the latter.  This was always bug, even
in the old code, but never witnessed because we ran so little software
in tickless mode.

Signed-off-by: Andy Ross <c70f9a6bf6ee0cd69c8af4ce5e6b131945769315@intel.com>

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