Currently, LLD may set both sh_link and sh_info for
.rela.plt section to zero when we have only .rela.iplt section part used.
ELF spec (https://docs.oracle.com/cd/E19683-01/816-1386/chapter6-94076/index.html)
says that for SHT_REL and SHT_RELA, sh_link references the associated symbol table
and sh_info the "section to which the relocation applies."
When we set the sh_link field, for the regular case we use the .dynsym index.
For .rela.iplt sections, it is unclear what is the associated symbol table,
because R_*_RELATIVE relocations do not use symbol names and we might have no
.dynsym section at all so this patch uses .symtab section index.
That helps to suppress the warnings generated by GNU Binutils.
The behavior is consistent with GNU linkers and at least does not violate
the spec it seems.