binutils ar uses -plugin to specify the LTO plugin, but LLVM doesn't
need this as it doesn't use a plugin for LTO. Accepting (and ignoring)
the option allows interoperability with existing build systems and
make downstream consumers life much easier.
No objections from Rafael on this change.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@273938 91177308-0d34-0410-b5e6-96231b3b80d8