Support nacl (Native Client) architecture

Introduce support for running on Native Client.

Native Client provides compatible os-independent,
but CPU-arch dependent binaries. The binary
created by this patch allows x86-64 machines
on any operating system supported by the NaCL
runtime to run the Docker client.

On machines with the Nacl SDK, the resultant binary
may be invoked via sel_ldr_x86_64.

While not yet fully implemented in this patch, the
Native Client also allows, with user-permission,
the Docker client to run inside of a Chromium browser.
Furthermore, this support may be used to bring the
Docker client and its packages into ZeroVM based
application unikernels.

Signed-off-by: Eric Windisch <[email protected]>