add merge strings option, this temporarily fixes the problem bringing up helloworld with glibc on x86_64

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@175038 91177308-0d34-0410-b5e6-96231b3b80d8
fix the mergeable string atom, when the target points to a symbol which is at a relative offset from the start of the .rodata section

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@175039 91177308-0d34-0410-b5e6-96231b3b80d8