[llvm-readobj] - Simplify .gnu.version_r dumping a bit.

Current implementation takes "Number of needed versions" from DT_VERNEEDNUM
dynamic tag entry. Though it would be a bit simpler to take it from sh_info
section header field directly:
https://docs.oracle.com/cd/E19683-01/816-1386/chapter6-94076/index.html

Differential revision: https://reviews.llvm.org/D58048

llvm-svn: 353814