[ELF] - Add missing dynamic tags when producing output with IRelative relocations only.

This is "Bug 35751 - .dynamic relocation entries omitted if output
contains only IFUNC relocations"

We have InX::RelaPlt and InX::RelaIPlt synthetic sections for PLT relocations.
They are usually live in rela.plt section. Problem appears when InX::RelaPlt
section is empty. In that case we did not produce normal set of dynamic tags
required, because logic was written in the way assuming we always have
non-IRelative relocations in rela.plt.

Patch fixes the issue.

Differential revision: https://reviews.llvm.org/D41592

llvm-svn: 321600