Define XER bits as a single register and access them individually to
avoid defining 5 32-bit registers (TCG doesn't permit to map 8-bit
registers).
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5500 c046a42c-6fe2-441c-8c8c-71466251a162