[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