Enable security checks for arc4random() on CloudABI as well.

CloudABI also supports the arc4random() function. We can enable compiler
warnings for rand(), random() and *rand48() on this system as well.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@231914 91177308-0d34-0410-b5e6-96231b3b80d8