Changes

[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