[llvm-readelf] - Improve ELF type field dumping.

This is related to https://bugs.llvm.org/show_bug.cgi?id=40868.

Currently we don't print `OS Specific`/``Processor Specific`/`<unknown>`
prefixes when dumping the ELF file type. This is not consistent
with GNU readelf. The patch fixes it.

Also, this patch removes the `types.test`, because we already have
`file-types.test`, which tests more cases and this patch revealed that
we have such a duplicate.

Differential revision: https://reviews.llvm.org/D93096