[llvm-readobj][test] - Stop using binaries in gnu-phdrs.test, split and refine it.

gnu-phdrs.test uses 2 precompiled objects and has issues:

* It does not test all possible program headers.
* It does not test target-specific headers (arm, mips).
* It does not use --strict-whitespace --match-full-line to check the format of the output.
* It is possible to check things better/nicer.
* It also tests --section-mapping.

This patch makes gnu-phdrs.test to stop using binaries and addresses issues above.
It splits --section-mapping testing to gnu-section-mapping.test

I am going to use it as a base for a follow-up patch that should also test
section to segment mapping conditions that we currently have
(see the discussion in D78709)

Differential revision: https://reviews.llvm.org/D78805