[Clang][NewPM] Don't bail out if the target machine is empty
This matches the behavior of the old pass manager. There are some
targets that don't have target machine at all (e.g. le32, spir) which
whose tests would never run with new pass manager. Similarly, we would
need to disable tests for targets that are disabled.
Differential Revision: https://reviews.llvm.org/D58374