cpuidle-haltpoll: Replace default_idle() with arch_cpu_idle()

When a KVM guest has MWAIT, mwait_idle() is used as the default idle
function.

However, the cpuidle-haltpoll driver calls default_idle() from
default_enter_idle() directly and that one uses HLT instead of MWAIT,
which may affect performance adversely, because MWAIT is preferred to
HLT as explained by the changelog of commit aebef63cf7ff ("x86: Remove
vendor checks from prefer_mwait_c1_over_halt").

Make default_enter_idle() call arch_cpu_idle(), which can use MWAIT,
instead of default_idle() to address this issue.

Suggested-by: Thomas Gleixner <[e-mail]>
Suggested-by: Rafael J. Wysocki <[e-mail]>
Signed-off-by: Li RongQing <[e-mail]>
[ rjw: Changelog rewrite ]
Signed-off-by: Rafael J. Wysocki <[e-mail]>

arch/x86/kernel/process.c

···
 {
 	x86_idle();
 }
+EXPORT_SYMBOL_GPL(arch_cpu_idle);
 
 /*
  * We use this if we don't have any better idle routine..

drivers/cpuidle/cpuidle-haltpoll.c

···
 		local_irq_enable();
 		return index;
 	}
-	default_idle();
+	arch_cpu_idle();
 	return index;
 }