[llvm-readobj/elf] - Add a testing for --stackmap and refine the implementation.

Currently, we only test the `--stackmap` option here:
https://github.com/llvm/llvm-project/blob/master/llvm/test/Object/stackmap-dump.test
it uses a precompiled MachO binary currently and I've found no tests for this option for ELF.

The implementation also has issues. For example, it might assert on a wrong version
of the .llvm-stackmaps section. Or it might crash on an empty or truncated section.

This patch introduces a new tools/llvm-readobj/ELF test file as well as implements a few
basic checks to catch simple crashes/issues

It also eliminates `unwrapOrError` calls in `printStackMap()`.

Differential revision: https://reviews.llvm.org/D85208