kernel - Increase size of savefpu area in pcb for vkernel64

kernel - Increase size of savefpu area in pcb for vkernel64

* The vkernel64 uses bcopy.  The ucontext_t/mcontext_t reserves a 1024
  byte space for the fpu area, but the savefpu structure only reserves
  834 bytes.  Increase the savefpu structure appropriately.

* Did not effect real 64-bit kernels because they used machine instructions
  to save/restore the FP state and/or bcopy'd using the savefpu structure
  sdize instead of the size of the fpu area in the ucontext/mcontext
  structure.

* Fixes panics in vkernel64

Reported-by: tuxillo