lxc uses uname to check the kernel version.  Seccomp respects userspace.  In the case
of 32-bit userspace on 64-bit kernel, this was a bad combination.

When we run into that case, make sure that the compat seccomp context is 32-bit, and
the lxc->seccomp_ctx is the 64-bit.

Closes #654
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>