[llvm-readobj] - Massive test cases cleanup.

This patch does the following:
1) Starts using YAML macro to reduce the number of YAML documents in tests.
2) Adds `#` before 'RUN'/`CHECK` lines in a few tests where it is missing.
3) Removes unused YAML keys.
4) Starts using `ENTSIZE=<none>` to simplify tests (see D84526).
5) Removes trailing white spaces in a few places.

Differential revision: https://reviews.llvm.org/D85013