fix thread migration code (i.e., sched_setaffinity())

- moved migration code into idle() process and updated schedule() to detect
  when a thread has moved to another CPU in order to avoid doing housekeeping
  on behalf of the original one
- start CPU head from core 0
- keeps track of nested interrupts

arch/x86/kernel/syscall.c

@@ -79,7 +79,7 @@ Core with BSP HW ID 226 boots next and is given  SW-ID of 226.
 Core with BSP HW ID 227 boots next and is given  SW-ID of 227.
 */
 static ihk_spinlock_t cpuid_head_lock = 0;
-static int cpuid_head = 1;
+static int cpuid_head = 0;
 
 /* archtecture-depended syscall handlers */
 int obtain_clone_cpuid() {