This patch moves `is_aborting()` and `is_halting()`
from `kernel/sched.c` to `kernel/include/kthread.h`
and renames them to `z_is_thread_aborting()` and `z_is_thread_halting()`,
for consistency with other internal kernel APIs.
It replaces the previous inline function definitions in `sched.c`
with calls to the new header functions. Additionally, direct bitwise
checks like `(thread->base.thread_state & _THREAD_DEAD) != 0U`
are updated to use the new `z_is_thread_dead()` helper function.
This enhances code readability and maintainability.
Signed-off-by: TaiJu Wu <tjwu1217@gmail.com>
e069ce242c