x86/cpuidle: really use C1 for "urgent" CPUs

x86/cpuidle: really use C1 for "urgent" CPUs

For one on recent AMD CPUs entering C1 (if available at all) requires
use of MWAIT, while HLT (i.e. default_idle()) would put the processor
into as deep as CC6. And then even on other vendors' CPUs we should
avoid entering default_idle() when the intended state can be reached
by using the active idle driver's facilities.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>