[ELF] Fix functionality of merging similar strings.

For PC relative accesses, negative addends were to be ignored. The linker was
not ignoring it and would fail with an assert. This fixes the issue and is able
to get Helloworld working.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@220179 91177308-0d34-0410-b5e6-96231b3b80d8