Add support for ARMv8 (AKA: arm64 or Aarch64)

[adconrad]

The commit itself should be self-explanatory. This was tested with docker 1.5 on Ubuntu with gccgo. Should work with golang as well, in theory, but we'll test that theory when the golang port is more complete.

[crosbymichael]

@adconrad code looks good but could you sign your commit?

These commits do not have a proper 'Signed-off-by:' marker:

• 9d07683

Please amend each commit to include a properly formatted DCO marker.

Visit the following URL for information about the Docker DCO:
https://github.com/docker/docker/blob/master/CONTRIBUTING.md#sign-your-work

[adconrad]

On Thu, Apr 09, 2015 at 09:32:56AM -0700, Michael Crosby wrote:

Visit the following URL for information about the Docker DCO:
https://github.com/docker/docker/blob/master/CONTRIBUTING.md#sign-your-work

Right, can do. I probably have some PPC fixes and an ARM fix to
commit as well, so I'll sort all of that later today, make sure
to sign them all, and re-propose the PR.

... Adam

[crosbymichael]

@adconrad thanks!