x86, hotplug: Fix powersavings with offlined cores on AMD

ea53069231f9317062910d6e772cca4ce93de8c8 made a CPU use monitor/mwait
when offline. This is not the optimal choice for AMD wrt to powersavings
and we'd prefer our cores to halt (i.e. enter C1) instead. For this, the
same selection whether to use monitor/mwait has to be used as when we
select the idle routine for the machine.

With this patch, offlining cores 1-5 on a X6 machine allows core0 to
boost again.

[ hpa: putting this in urgent since it is a (power) regression fix ]

Reported-by: Andreas Herrmann <[email protected]>
Cc: [email protected] # 37.x
Cc: H. Peter Anvin <[email protected]>
Cc: Arjan van de Ven <[email protected]>
Cc: Len Brown <[email protected]>
Cc: Venkatesh Pallipadi <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
LKML-Reference: <[email protected]>
Signed-off-by: H. Peter Anvin <[email protected]>

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