[SystemZ] Update README


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188062 91177308-0d34-0410-b5e6-96231b3b80d8
Add back missing PPC relocation types.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188064 91177308-0d34-0410-b5e6-96231b3b80d8