Fix _CPU_SMP_Get_current_processor(), since read from register MPIDR_EL1
(the least significant byte) is not always right,for example the least
significant byte of MPIDR_EL1 is always zero for cortex-a55.
aarch64 has a thread ID register TPIDR_EL1 available for for OS management
purposes. RTEMS stores per-CPU control in TPIDR_EL1 when the core startup,
so we can use _Per_CPU_Get_index() to get current processor index.
update #5064
5e5214b786