sched/idle: Move the cpuidle entry point to the generic idle loop

In order to integrate cpuidle with the scheduler, we must have a better
proximity in the core code with what cpuidle is doing and not delegate
such interaction to arch code.

Architectures implementing arch_cpu_idle() should simply enter
a cheap idle mode in the absence of a proper cpuidle driver.

In both cases i.e. whether it is a cpuidle driver or the default
arch_cpu_idle(), the calling convention expects IRQs to be disabled
on entry and enabled on exit. There is a warning in place already but
let's add a forced IRQ enable here as well.  This will allow for
removing the forced IRQ enable some implementations do locally and
allowing for the warning to trig.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Preeti U Murthy <preeti@linux.vnet.ibm.com>
Cc: Paul Mundt <lethal@linux-sh.org>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Cc: Olof Johansson <olof@lixom.net>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/alpine.LFD.2.11.1401291526320.1652@knanqh.ubzr


Signed-off-by: Ingo Molnar <mingo@kernel.org>