Fix some review issues.

kernel/src/arch/mipsel/compiler_rt.rs

@@ -1,8 +0,0 @@
-//! Workaround for missing compiler-builtin symbols
-//!
-//! [atomic](http://llvm.org/docs/Atomics.html#libcalls-atomic)
-
-#[no_mangle]
-pub extern fn abort() {
-    panic!("abort");
-}

kernel/src/arch/mipsel/context.rs

@@ -50,8 +50,8 @@ pub struct TrapFrame {
     pub sp: usize,
     pub fp: usize,
     pub ra: usize,
-    /// Reserve space for hartid
-    pub _hartid: usize,
+    /// Reserved
+    pub reserved: usize,
 }
 
 impl TrapFrame {

kernel/src/arch/mipsel/cpu.rs

@@ -1,4 +1,5 @@
 use mips::registers::cp0;
+use mips::instructions;
 use crate::consts::MAX_CPU_NUM;
 use core::ptr::{read_volatile, write_volatile};
 
@@ -21,7 +22,7 @@ pub unsafe fn start_others(hart_mask: usize) {
 }
 
 pub fn halt() {
-    /* nothing to do */
+    unsafe { instructions::wait(); }
 }
 
 pub unsafe fn exit_in_qemu(error_code: u8) -> ! {