ppc64: Fix buggy floating point register handling

The checkpoint and restore of the Power PC floating point registers is
buggy.

The issue is that the signal frame context is defined to store double value
while the protocol buffer is handling unsigned 64bits integer value. A
silent cast done by the compiler was modifying the restored value in our
back.

This fix changes the type used when manipulating the FP registers value to
be consistent between checkpoint and restart.

Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>