Auto merge of #2369 - alexcrichton:no-more-ar, r=alexcrichton

This is largely no longer needed as we bundle llvm-ar and use that (and it's
cross platform). Note that the underlying support still exists in Cargo as the
flag hasn't been removed from the compiler outright, and older compilers may
still be in use. Just no need to document it so prominently when it's no longer
needed!