[llvm-readobj] - Dump the ELF file type better.

Currently llvm-readelf might print "OS Specific/Processor Specific/<unknown>"
hint when dumping the ELF file type. The patch teaches llvm-readobj to do the same.

This fixes https://bugs.llvm.org/show_bug.cgi?id=40868

I am removing `Object/elf-unknown-type.test` test because it is not in the right place,
it is outdated and very limited.
The `readobj/ELF/file-types.test` checks the functionality much better.

Differential revision: https://reviews.llvm.org/D93689