target-ppc: implement darn instruction

darn: Deliver A Random Number

Currently return invalid random number for all the case. This needs
proper algorithm to provide cryptographically suitable random data.
Reading from /dev/random can block and that is not an expected behaviour
while the cpu instruction is getting executed. Moreover, /dev/random
would only work for linux-user

Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.vnet.ibm.com>
Signed-off-by: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>
[dwg: Added minor clang warning fix for ppc32 target]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
