[yaml2obj] - Make .symtab to be not mandatory section for SHT_REL[A] section.

Before this change .symtab section was required for SHT_REL[A] section
declarations. yaml2obj automatically defined it in case when YAML document
did not have it.

With this change it is now possible to produce an object that
has a relocation section, but has no symbol table.

It simplifies the code and also it is inline with how we handle Link fields
for another special sections.

Differential revision: https://reviews.llvm.org/D69260