Remove the 'N' modifier from llvm-ar.

* It is not present on OS X.
* It is untested.
* It is not needed for using ar in a build system.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186080 91177308-0d34-0410-b5e6-96231b3b80d8