[llvm-readelf] - Make GNU style dumping of invalid SHT_GNU_verdef be consistent with LLVM style.

When we dump SHT_GNU_verdef section that has sh_link that references a non-existent section,
llvm-readobj reports a warning and continues dump, but llvm-readelf fails with a error.

This patch fixes the issue and opens road for futher follow-ups for
improving the printGNUVersionSectionProlog().

Differential revision: https://reviews.llvm.org/D70776