debug: arc: expose thread relinquish cause

When a thread is preempted on ARC, caller-saved registers
are saved on the stack automatically (RIRQ) or manually
by the kernel ISR (FIRQ).
Exposing the thread's relinquish cause hints the debugger
that it can read more or less register values.

Signed-off-by: Bruno Mendes <bd_mendes@outlook.com>