self.path may be empty or otherwise miss the normal system directories,
so try PATH next. Assume it is sane enough to cover the usual system
bash locations too, but the old list is not good enough for NetBSD.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193471 91177308-0d34-0410-b5e6-96231b3b80d8