Refactor code related to CPU local memory

Co-authored-by: Chuandong Li <lichuand@pku.edu.cn>

ostd/src/cpu/mod.rs

@@ -0,0 +1,12 @@
+// SPDX-License-Identifier: MPL-2.0
+
+//! CPU-related definitions.
+
+pub mod cpu_local;
+
+cfg_if::cfg_if! {
+    if #[cfg(target_arch = "x86_64")]{
+        pub use trapframe::GeneralRegs;
+        pub use crate::arch::x86::cpu::*;
+    }
+}