Do not emit a corrupt symbol table entry for .rela_iplt_{start,end}.

If .rela.iplt does not exist, we used to emit a corrupt symbol table
that contains two symbols, .rela_iplt_{start,end}, pointing to a
nonexisting section.

This patch fixes the issue by setting section index 0 to the symbols
if .rel.iplt section does not exist.

Differential Revision: https://reviews.llvm.org/D56623

llvm-svn: 351218