sched: remove unnecessary reschedule_from_interrupt() parameter.

Before commit 202b2cccc14a43d088002a80f7add51e3e4bc4ce,
cpu::reschedule_from_interrupt() needed to know whether we were called
from an interrupt (preempt=true) or as an ordinary function (preempt=false),
to know whether or not to save the FPU state.

As we now save the FPU state at the interrupt code,
reschedule_from_interrupt() no longer needs to deal with this, and so this
patch removes the unneeded paramter "preempt" to that function.

One thing we are losing in this patch is the "sched_preempt" tracepoint,
which we previously had when an interrupt caused an actual context switch
(not just a reschedule call, but actually switching to a different thread).
We still have the "sched_switch" tracepoint which traces all the context
switches, which is probably more interesting anyway.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>