kernel: smp: CPU start may result in null pointer access

It is observed that starting up CPU may result in other CPUs
crashing due to de-referencing NULL pointers. Note that this
happened on the up_squared board, but there was no way to
attach debugger to verify. It started working again after
moving z_dummy_thread_init() before smp_timer_init(), which
was the old behavior before commit
eefaeee0 where the issue
first appeared. So mimic the old behavior to workaround
the issue.

Fixes #68115

Signed-off-by: Daniel Leung <daniel.leung@intel.com>