[llvm-readobj] - Add a validation of the GNU hash table to printGnuHashHistogram().

Similar to D81937, we might crash when printing a histogram for a GNU hash table
with a 'symndx' index that is larger than the number of dynamic symbols.

This patch adopts and reuses the `getGnuHashTableChains()` helper which performs
a validation of the table. As a side effect the warning reported for
the --gnu-hash-table was improved.

Also with this change we start to report a warning when the histogram is requested for
the GNU hash table, but the dynamic symbols table is empty (size == 0).

Differential revision: https://reviews.llvm.org/D82010