Allow 32-on-64 execution for LXC guests

Allow 32-on-64 execution for LXC guests

Using the 'personality(2)' system call, we can make a container
on an x86_64 host appear to be i686. Likewise for most other
Linux 64bit arches.

* src/lxc/lxc_conf.c: Fill in 32bit capabilities for x86_64 hosts
* src/lxc/lxc_container.h, src/lxc/lxc_container.c: Add API to
  check if an arch has a 32bit alternative
* src/lxc/lxc_controller.c: Set the process personality when
  starting guest