score: Fix TLS size usage
doc: Add SMP glossary
score: SMP_FATAL_SCHEDULER_WITHOUT_PROCESSORS

Avoid the SMP_FATAL_SCHEDULER_WITHOUT_PROCESSORS fatal error and make it
a run-time error in rtems_scheduler_ident() and _Scheduler_Get_by_id().
score: Add SMP test message handler

This handler can be used to test the inter-processor interrupt
implementation.
score: Use atomic operations for SMP messages