Scheduler preemption hooks, #2

I redid the the scheduler preemption patches based on Rusty's feedback.
Things do look much cleaner this time around.

Changes:
- split into core and kvm parts
- incorporated Shaohua Li's fix
- each task now has a list of independently managed preemption hooks
- move preempt hook pointer to a hot cache line within task_struct

I haven't made kernel_fpu_begin()/kernel_fpu_end() preemptible as promised,
because (a) we need to find a place to put the fpu state, and (b) we should
get better performance by reusing cr0.ts anyway.


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