kernel: thread: mark return undefined in z_thread_entry using DWARF

Add DWARF hint to handle z_thread_entry correctly
in debuggers. This function starts a new thread and never returns.
Use `.cfi_undefined` so DWARF-based unwinding does not rely on return.
Without this, unwinding may follow a bogus return address, leading to
invalid memory reads and potential bus faults during backtrace.

Signed-off-by: Peter van der Perk <peter.vanderperk@nxp.com>