If you aren't familiar with Linaro it is the Android optimization that promises 100% better performance than stock Google Android. It was first seen on 4.0.4 and is now making further advancements for future device. We don't have any 64-bit ARM devices for public consumption just yet, but Linaro has already released a build of Kit Kat that includes 64-bit compatibility. Companies like Qualcomm and Marvell have already announced plans to release 64-bit processors which is probably what Linaro used to test their builds prior to release. We may see a 64-bit version of Android as soon as June when it could be announced at Google I/O. For a full in depth explanation of Linaro on 64-bit devices head to the source link below
Via AndroidAuthority