[llvm-readobj] - Do not skip building of the GNU hash table histogram.

When the `--elf-hash-histogram` is used, the code first tries to build
a histogram for the .hash table and then for the .gnu.hash table.

The problem is that dumper might return early when unable or do not need to
build a histogram for the .hash.

This patch reorders the code slightly to fix the issue and adds a test case.

Differential revision: https://reviews.llvm.org/D80204