kernel - Make the x86-64 double-fault exception operational

kernel - Make the x86-64 double-fault exception operational

* Finish implementing the double-fault exception.  Only cpu0 implemented
  the proper interrupt stack configuration before.  Now all cpus implement
  the proper interrupt stack.

* For x86-64 use the idle thread stack for the double-fault exception
  stack for now (probably not the best idea but better than nothing).

* For both i386 and x86-64, when a double fault occurs, report whether
  it is a generic double fault or whether it is due to a kernel stack
  guard.