coreos-kernel: enable X32 ABI

This only enables kernel support for the [X32 ABI][1], which is a useful optimisation. A program compiled to this ABI will run in native 64-bit mode, but with 32-bit pointers and data values.

[1]: http://lwn.net/Articles/456731/

There would need to be more work needed, if someone wants to enable it in the CoreOS SDK or even optimise CoreOS's memory footprint even further by compiling systemd to use X32 ABI. I don't think cgo currently supports X32, but one could probably try compiling Docker with gccgo to reduce his footprint (might be an interesting project).