Because of Linux's calling convention, it should not be necessary to
save the FPU state when a reschedule is caused by a function call.

Because we had a bug and forgot to save the FPU state when calling
a signal handler, and because this signal handler can cause a reschedule,
we had to save the FPU on any reschedule. But after fixing that bug, we
no longer need these unnecessary FPU saves.

The "sunflow" benchmark still runs well after this patch.