SMP: Fix start multitasking for some targets

The previous SMP multitasking start assumed that the initial heir thread of a
processor starts execution in _Thread_Handler().  The _Thread_Handler() sets
the interrupt state explicitly by _ISR_Set_level() before it calls the thread
entry.  Under certain timing conditions, processors may perform an initial
context switch to a thread which already executes its thread body (see
smptests/smpstart01).  In this case, interrupts are disabled after the context
switch on targets which do not save/restore the interrupt state during a
context switch (aarch64, arm, and riscv).

Close #4627.

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