Move sysfs cpu hotplug labeling to genfs_contexts

On Marlin ~120 ms of time is spent relabeling /sys/devices/system/cpu
every time we come out of suspend. Moving from file_contexts to
genfs_contexts as the labeling mechanism knocks this down to ~3 ms.

Bug: 32938130
Test: build and boot Marlin. Verify that files in
    /sys/devices/system/cpu have the proper label before and after
    suspend.

Change-Id: Ie71ea7e3dd5df250cabe4ba9600afbf67e69f720