MFC r332918, r333222: go deeper for ACPI suspend bounce test

MFC r332918, r333222: go deeper for ACPI suspend bounce test

debug.acpi.suspend_bounce sysctl now allows a deeper dive into the sleep
abyss.  The system will execute the suspend sequence up to the call to
AcpiEnterSleepState().  That includes saving processor contexts and
parking APs.  Then, instead of actually entering the sleep state, the
BSP will call resumectx() to emulate the wakeup.  The APs should get
restarted by the sequence of Init and Startup IPIs that BSP sends to
them.

AcpiOsEnterSleep() is used to implement this feature.

Joint work with jkim.