kernel/smp: Fix bitrot with the way the SMP "start flag" works

Tickless timers mean that k_busy_wait() won't work until after the
timer driver is initialized, which is very early but not as early as
SMP.  No need for it, just spin.

Also the original code used a stack variable for the start flag, which
racily presumed that _arch_start_cpu() would comes back synchronously
with the other CPU fired up and running right now.  The cleaned up smp
bringup API on x86_64 isn't so perky, so it exposed the bug.  The flag
just needs to be static.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>