kernel - Fix mid-kernel-boot lockups due to module order

kernel - Fix mid-kernel-boot lockups due to module order

* Sometimes the kernel would just lock up for no apparent reason.  Changing
  the preloaded module set (even randomly) could effect the behavior.

* The problem turned out to be an issue with kernel modules needing to
  temporarily migrate to another cpu (such as when installing a SWI or
  other interrupt handler).  If the idle thread on cpu 0 had not yet
  bootstrapped, lwkt_switch_return() would not be called properly and
  the LWKT cpu migration code would never schedule on the target cpu.

* Fix the problem by handling the idle thread bootstrap case for cpu 0
  properly in pc64.