[ELF] Fix uninitalized value.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@191924 91177308-0d34-0410-b5e6-96231b3b80d8
[ELF] Add explictly exported atoms and export R_*_COPY'ed atoms.
[ELF][x86-64] Generate PLT for R_*_32S relocs.
[ELF][x86-64] Treat R_*_32 as R_*_32S for this pass.
[ELF] Refactor x86-64 relocation pass.

This renames the GOTPLTPass to RelocationPass and refactors it to better
represent the different types of relocations.
[ELF] Implement .{init,fini}_array ordering.
[ELF] Implement minimal support for .eh_frame_hdr.