[ELF] Add ELFTargetRelocationHandler.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@173896 91177308-0d34-0410-b5e6-96231b3b80d8
[ELF][x86-64] Add X86_64TargetRelocationHandler and friends.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@173897 91177308-0d34-0410-b5e6-96231b3b80d8