[lit] Avoid CRLFs in bash scripts on Windows

Native Windows Python will do line ending translation by default, which
we don't want in bash scripts.  If we're not native Windows Python, then
'b' is ignored.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177602 91177308-0d34-0410-b5e6-96231b3b80d8