[llvm-readobj][llvm-objdump] - Cleanup testing of dynamic tags dumping.

We have the `elf-dynamic-tags-machine-specific.yaml` input shared
between the llvm-readobj and llvm-objdump test.
It looks strange, because tools usually does not share inputs.

Also there are following problems related:
1) `elf-dynamic-tags-machine-specific.yaml` input contains excessive YAML parts.
2) objdump's test case never test AARCH64 tags.
3) There are unknown tags in the `elf-dynamic-tags-machine-specific.yaml` and
    `dynamic-tags-machine-specific.test`, though we already testing unknown tags
    in `\llvm-readobj\ELF\dynamic-tags.test` and `llvm-objdump\elf-dynamic-section.test` tests.

This patch removes the shared input and refines the test cases to resolve
issues mentioned.

Differential revision: https://reviews.llvm.org/D71602