[ELF] - Tweak verneed.s test to use new llvm-readobj functionality

Previously this test performed check of binary data. Since
llvm-readobj currently able to dump all 3 types of version relative
sections, that can be used to make this test more transparent.
Patch do that.

Differential revision: http://reviews.llvm.org/D21024

llvm-svn: 272120