Enable ARM builds for my xenial build tests PPA

The PPA is here: https://launchpad.net/~teward/+archive/ubuntu/xenial-buildtests

This PPA is usually used for build-testing of my merges for the Xenial cycle of the nginx package. These packages first undergo local builds on my system via sbuild for amd64 and i386, however I don't have a stable ARM build environment. After it succeeds without issues in my local builders several times (I test every time i have a potentially viable merge), I would be uploading the final 'preliminary' copy to the PPA. This results in only a few uploads for this PPA, but there has been weirdness in the past with ARM-specific things. As I don't have a specific ARM builder in my area to test build these under ARM, I'd like to be able to have an ARM build available as a 'it builds!' proof.

We're looking at very sparse uploads for this, but enough that there will be one or two a month, until the Xenial release when the PPA goes away.