Change getRelocationAdditionalInfo to be ELF only.

It was only implemented for ELF where it collected the Addend, so this
patch also renames it to getRelocationAddend.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181502 91177308-0d34-0410-b5e6-96231b3b80d8