Make idle loop not miss a chance to schedule

Prevents the case where McKernel misses one chance to schedule a
task when an interrupt occurs just after enabling interrupt and
just before executing "halt" in the idle loop (in kernel/process.c).

refs #45

lib/include/ihk/cpu.h

@@ -19,6 +19,7 @@
 void cpu_enable_interrupt(void);
 void cpu_disable_interrupt(void);
 void cpu_halt(void);
+void cpu_safe_halt(void);
 void cpu_restore_interrupt(unsigned long);
 void cpu_pause(void);