FPU support.

Obtained from: NetBSD (portions)

Make fork work, at least for kthreads. Switching still has some issues.

Save WIP. Partial rewrite of cpu_switch() and savectx(). This makes it closer
to working but still needs some work to properly switch the full context
(such as saving the fpu registers, switch stacks

Save WIP. Partial rewrite of cpu_switch() and savectx(). This makes it closer
to working but still needs some work to properly switch the full context
(such as saving the fpu registers, switch stacks, etc.). Also, remove some
dead code that was mixed in.

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Bring in NetBSD code used in the PowerPC port.

Reviewed by: obrien, dfr
Obtained from: NetBSD