Add PowerPC release notes for 3.5.

Here's my take on 3.5 changes for PowerPC.  Others please feel free to add,
edit, delete as desired.

Thanks,
Bill

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214403 91177308-0d34-0410-b5e6-96231b3b80d8