[ELF] Make updateReferenceForMergeStringAccess virtual.
[ELF] Make updateReferenceForMergeStringAccess virtual.

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

git-svn-id: f6089bf0e6284f307027cef4f64114ee9ebb0424@220180 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld