nm: fall back to heuristic when ELF symbol has zero size

nm: fall back to heuristic when ELF symbol has zero size

Size being set for a symbol isn't a strict requirement in ELF. For ones
not having their size set, fall back to the same logic as used for non-
ELF, non-COFF symbols.

While there switch to using elf_symbol_from() instead of kind of open-
coding it.