llvm: Get rid of -Wunused-const-variable warnings.

Often the thread k_tid_t value is not referenced for
self-standing threads, and should therefore be allowed
to remain unused.

Signed-off-by: Jaagup Averin <[email protected]>

Author: JaagupAverin

include/zephyr/kernel.h

@@ -805,7 +805,7 @@ struct _static_thread_data {
 				     _k_thread_stack_##name, stack_size,\
 				     entry, p1, p2, p3, prio, options,	\
 				     delay, name);			\
-	const k_tid_t name = (k_tid_t)&_k_thread_obj_##name
+	__maybe_unused const k_tid_t name = (k_tid_t)&_k_thread_obj_##name
 
 /**
  * INTERNAL_HIDDEN @endcond