[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
[ELF] Make updateReferenceForMergeStringAccess virtual.

The ELF subtargets would usually want to override the function
updateReferenceForMergeStringAccess. Allow this by making it virtual.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@220180 91177308-0d34-0410-b5e6-96231b3b80d8