ELF: emit symbol table when there are relocations

Even when there are no symbols (e.g. all relocations being against
absolute values), a symbol table (with just the first placeholder entry)
needs to be emitted. Otherwise tools like objdump won't properly process
the relocations. The respective checks in assign_section_numbers() and
_bfd_elf_compute_section_file_positions() support also this view. Oddly
enough so far HAS_RELOC was only set when reading in an object file, but
not when generating one anew; the flag would only have been cleared when
no relocations were found (anymore).

While there also amend the affected function's leading comment to also
mention gas.