ppc64: fixing comment

Fixing comment leading to think that 32bit tasks are supported, which is
not the case.

For the record, ppc64le is not supporting 32bit task, while ppc64 (the Big
Endian architecture) has an option to support 32bit task but CRIU doesn't
yet run on ppc64.

Reported-by: Christopher Covington <cov@codeaurora.org>
Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>