powerpc: Added __cmpdi2 for signed 64bit comparision

This was missing on powerpc and I am getting compilation error
drivers/vfio/pci/vfio_pci_rdwr.c:193: undefined reference to `__cmpdi2'
drivers/vfio/pci/vfio_pci_rdwr.c:193: undefined reference to `__cmpdi2'

Signed-off-by: Bharat Bhushan <bharat.bhushan@freescale.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>