[llvm-readobj] Provide Hash Histogram for all ELFDumper implementations

Previously, the GNUELFDumper was the only implementer for Hash Histograms.
This patch moves the implementation for printHashHistogram and
printGnuHashHistogram into the ELFDumper base class, and allows each
derived class to specialize how it outputs that information.

This change also prevents the JSONELFDumper from emitting invalid JSON,
since the shared implementation in LLVMELFDumper no longer emits a
warning message to the output stream.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D137096