test/hello_world: use P9MAMBO_ to differentiate from P8
test/hello_world: use P9MAMBO_ to differentiate from P8

Helps if you're building mambo from source (or haven't used the
packages in exactly the way they install) to ensure you run both
the p8 and p9 mambo simulators for testing.

Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.vnet.ibm.com>

qemu/skiboot,open-power/skiboot,stewart-ibm/skiboot,open-power/skiboot,legoater/skiboot,legoater/skiboot,shenki/skiboot,mikey/skiboot,open-power/skiboot,stewart-ibm/skiboot,stewart-ibm/skiboot,mikey/skiboot,open-power/skiboot,qemu/skiboot,shenki/skiboot,legoater/skiboot,legoater/skiboot,qemu/skiboot,shenki/skiboot,qemu/skiboot,shenki/skiboot,qemu/skiboot,mikey/skiboot,shenki/skiboot,legoater/skiboot,open-power/skiboot