[ELF] Fallback to sh_link=0 if neither .dynsym nor .symtab exists

Summary: .rela.plt may only contain R_*_{,I}RELATIVE relocations and not need a symbol table link. bfd/gold fallbacks to sh_link=0 in this case. Without this patch, ld.lld --strip-all caused lld to dereference a null pointer.

Reviewers: ruiu, grimar, espindola

Reviewed By: ruiu

Subscribers: emaste, arichardson, llvm-commits

Differential Revision: https://reviews.llvm.org/D53881

llvm-svn: 345648