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