Collect the Addend for external relocs.

This fixes 2013-04-04-RelocAddend.ll. We don't have a testcase for non external
relocs with an Addend. I will try to write one.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@180767 91177308-0d34-0410-b5e6-96231b3b80d8