target/ppc: introduce the PPC_BIT() macro
    
    and use them in a couple of obvious places. Other macros will be used
    in the model of the XIVE interrupt controller.
    
    Signed-off-by: Cédric Le Goater <clg@kaod.org>
    Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
    (cherry picked from commit 2a83f9976efa9a85e8ceb9d1035a68f25c321334)
    Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>