[llvm-readobj] - Refine logic of the symbol table locating in printRelocationsHelper().

This removes the last `unwrapOrError` call from the `printRelocationsHelper`.

There is a little additional complexity because of `SHT_RELR/SHT_ANDROID_RELR` sections.
Such sections contains only relative relocations and they do not have a
symbol table associated with them, hence we should not try to treat
their `sh_link` field as a reference to a symbol table.

Differential revision: https://reviews.llvm.org/D85430