Hey!
The work on 64-bit support has been progressing really well so it will be released in the next stable, scheduled 2016-04-04 (in two weeks). However we have had some trouble making a 64-bit version of Android-bundling (aapt), but are looking at shipping the 32-bit version for now. So in order to bundle for Android, you would need to enable multi-architecture support. Otherwise when bundling for Android, it would tell you that aapt could not be found, i.e. no 64-bit support.
Since this is brand new platform support and linux is quite hard to test for because of distro/env diversity, we would love to get some help in doing this. So if you feel like helping out, we are looking to make an alpha version available for external testing this week. We would greatly appreciate it!
A note if you want to alpha test. We are releasing Defold in different channels, each channel checks for updates happening to that channel. This alpha version would be on a specific channel, that would be abandoned after the linux 64-bit support has been shipped into the stable channel (i.e. the regular download-link in the dashboard). This means there will be no more updates to that alpha version. In order to jump on to the stable version and receive updates, you would need to download that version from the regular download link. When you do, it’s important that you sync (File > Synchronize) your game projects so that you bring over the stuff you made while using the alpha version. Another option is obviously to dig out the files in the FS and move them manually. 
I feel like this is a terrible explanation of what’s going on, let me know if this paragraph made no sense at all. 