Simplify the way we track the FPU state, using powerpc64 as a model.

Simplify the way we track the FPU state, using powerpc64 as a model.
The new code still uses the clean/dirty state that the hardware reports
to optimize saving/restoring the FPU register, but no longer attempts to
keep the FPU registers alive across a context switch.  Fixes panics seen
on MP kernels.

ok drahn@