- Generally improve register usage in cpu_switch.  Use the 'in' registers   for temporaries relating to the state of the new process instead of the   outs, so that functions can be called without fear of clobbering them. - Use savefpctx instead of rolling our own.
- Generally improve register usage in cpu_switch.  Use the 'in' registers
  for temporaries relating to the state of the new process instead of the
  outs, so that functions can be called without fear of clobbering them.
- Use savefpctx instead of rolling our own.

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