Teach kgdb how to handle double fault frames on i386:

- Save td_oncpu in 'struct kthr' so the i386 target code can see which CPU
  a thread is running on.
- Add a new frame unwinder for double fault frames.  This unwinder is used
  when "dblfault_handler" is encountered in the stack.  It uses the CPU of
  the current thread to lookup the base address of the TSS used for the
  double fault from the GDT.  It then fetches the various registers out
  of the TSS similar to how the current trapframe unwinder fetches
  registers out of the trapframe.

MFC after:	3 days

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