aarch64 current_task_ptr use sp_el0

[lhw2002426]

When I looked at the code of ruxos task ,I found in modules/ruxhal/src/cpu.rs,ruxos seems use sp_el0 to save current cpu ptr in aarch64. I think it is not reasonable.Why? I guess it may be a mistake.

/// Sets the pointer to the current task with preemption-safety.
///
/// Preemption may be enabled when calling this function. This function will
/// guarantee the correctness even the current task is preempted.
///
/// # Safety
///
/// The given `ptr` must be pointed to a valid task structure.
#[inline]
pub unsafe fn set_current_task_ptr<T>(ptr: *const T) {
   #[cfg(target_arch = "x86_64")]
   {
       CURRENT_TASK_PTR.write_current_raw(ptr as usize)
   }
   #[cfg(any(target_arch = "riscv32", target_arch = "riscv64"))]
   {
       let _guard = kernel_guard::IrqSave::new();
       CURRENT_TASK_PTR.write_current_raw(ptr as usize)
   }
   #[cfg(target_arch = "aarch64")]
   {
       use tock_registers::interfaces::Writeable;
       aarch64_cpu::registers::SP_EL0.set(ptr as u64)
   }
}
   ```