Add a dummy subtarget to the CPP backend target machine. This will
allow us to forward all of the standard TargetMachine calls to the
subtarget and still return null as we were before.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214727 91177308-0d34-0410-b5e6-96231b3b80d8