X32 ABI (x32 application binary interface) is an application binary interface project and one of the interfaces of the Linux kernel. It that allows programs to take advantage of the benefits of x86-64 (larger number of CPU registers, better floating-point performance, faster position-independent code shared libraries, function parameters passed via registers, faster syscall instruction) while using 32-bit pointers and thus avoiding the overhead of 64-bit pointers.
The x32 ABI was merged into the Linux kernel for the 3.4 release with support being added to the GNU C Library in version 2.16.
As of 23 February 2015 x32 is initialized in pld builders and packages can be built. You need rpm 5.4.15-12 at least to build x32 guest (chroot install).
Here's some random fixes that you may find useful.
vserver NAME build --context NUMBER -m poldek -n NAME --machine x32 -- -d pld-th
rm /etc/vserver/NAME/uts/machine as uname -m is supposed to return x86_64