score: Fix race condition in SMP startup

Do not use the Per_CPU_Control::started in
_SMP_Start_multitasking_on_secondary_processor() since this field may be
not up to date when a secondary processor reads it.  Use the read-only
scheduler assignment instead.

Add a new fatal error SMP_FATAL_MULTITASKING_START_ON_INVALID_PROCESSOR.
This prevents out-of-bounds access.

It is currently not possible to test these fatal errors.  One option
would be to fake values of the _CPU_SMP_Get_current_processor(), but
unfortunately this function is inline on some architectures.