Update PowerPC links in CompilerWriterInfo.rst

This updates the current references to links that work for me.
In the future, we should update the list of references itself to provide
information on newer architecture variants.

Thanks to Sean Silva for pointing out that the current links were broken!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174739 91177308-0d34-0410-b5e6-96231b3b80d8