[llvm-objcopy] - Do not crash on object that has relocations but no symbol table.

It was revealed by D69260.

Tool crashed when scanned relocations in a object without a symbol table.
This patch teaches it either to handle such objects (when relocations
does not use symbols we do not need a symbol table to proceed)
or to show an appropriate error otherwise.

Differential revision: https://reviews.llvm.org/D69304