[MCJIT][SystemZ] Use a simpler expression for indirect relocation offsets.

The expressions 'Reloc.Addend - Addend' and 'Reloc.Offset' should always be
equal in this context. The latter is prefered - we want to remove the
RelocationValueRef::Addend field in the future.





git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216418 91177308-0d34-0410-b5e6-96231b3b80d8