Driver: unify addClangRT{Linux,Windows}

The differences are pretty superficial:
- .lib vs .a extensions
- whether or not to link (potentially) incorrectly against libgcc_s

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224975 91177308-0d34-0410-b5e6-96231b3b80d8