[llvm-readobj] - Simplify .gnu.version_d dumping.

This is similar to D58048.

Instead of scanning the dynamic table to read the
DT_VERDEFNUM, we could take it from the sh_info field.
(https://docs.oracle.com/cd/E19683-01/816-1386/chapter6-94076/index.html)

The patch does this.

llvm-svn: 354270