[ELF] Use the target's LayoutHandler.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@173966 91177308-0d34-0410-b5e6-96231b3b80d8
[ELF] Fix circular initialization bug.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@173968 91177308-0d34-0410-b5e6-96231b3b80d8