[llvm-readobj/llvm-readelf] - Improve dumping of versioning sections.

Our elf-versioninfo.test is not perfect. It does not properly test how
flags are dumped and also we have a bug: they are dumped as enums in
LLVM style now, i.e not dumped properly.

GNU style uses a `versionFlagToString` method to build a string from flags
which seems is consistent with GNU readelf.

In this patch I fixed the issues mentioned.

Differential revision: https://reviews.llvm.org/D70399