Driver: Do not warn about unused -pthread when linking on darwin

While there is nothing to do at link time to get pthreads support on
darwin, specifying the argument is fine and we should not warn about
unused arguments.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@294065 91177308-0d34-0410-b5e6-96231b3b80d8