ppc64: Fix checkpoint --leave-running

The commit ba743509 which introduces support for Altivec and VSX
support was breaking the checkpoint --leave-running.

The root cause is that the address of the Altivec and VSX registers in the
signal frame should be computed for the stack in the context of the
checkpointed process.

This patch fixes this issue through the sigreturn_prep_fpu_frame which is
designed to update the signal frame based on the remote address.

Fixes: ba743509 ("ppc64: Add Altivec and VSX support")
Reported-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Tested-by: Andrew Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>