[ELF] Refactor x86-64 relocation pass.

This renames the GOTPLTPass to RelocationPass and refactors it to better
represent the different types of relocations.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@193379 91177308-0d34-0410-b5e6-96231b3b80d8