[elfabi] Fix a bug when .dynsym contains no non-local symbol

This patch fixed a bug when elbabi was supplied with a tbe file
contains no non-local symbol. Before this patch, it wrote 0 to
sh_info of the .dynsym section, making the ELF stub file invalid.
This patch fixed this issue.

Differential Revision: https://reviews.llvm.org/D96930