Release Notes: Overriding PPC64 and PPC64LE ABI defaults is not yet supported.

I wrongly included a description of a patch that came in after 3.5 branched
and has not been backported.

Thanks,
Bill

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214404 91177308-0d34-0410-b5e6-96231b3b80d8