Add checkpoint and restore of the Altivec and VSX registers.

Currently we rely on the return value of ptrace to detect if the CPU is
supporting these features or not. In the future, we should rely on the
AT_HWCAP vector and check feature at restart time.
Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>