Fix multiple issues pointed out by the new compiler

ostd/src/cpu/local/cell.rs

@@ -208,6 +208,7 @@ impl<T: 'static + SingleInstructionBitXorAssign<T>> CpuLocalCell<T> {
     ///
     /// Note that this memory operation will not be elided or reordered by the
     /// compiler since it is a black-box.
+    #[allow(unused)]
     pub fn bitxor_assign(&'static self, rhs: T) {
         let offset = self as *const _ as usize - __cpu_local_start as usize;
         // SAFETY: The CPU-local object is defined in the `.cpu_local` section,

ostd/src/cpu/local/single_instr.rs

@@ -114,6 +114,7 @@ pub trait SingleInstructionBitXorAssign<Rhs = Self> {
     /// # Safety
     ///
     /// Please refer to the module-level documentation of [`self`].
+    #[allow(unused)]
     unsafe fn bitxor_assign(offset: *mut Self, rhs: Rhs);
 }