unions `union_elem: ...', and use c99 syntax `.union_elem = ...' only
where necessary.
in this case, there's no need to tag elf_probe_func because that's the
first union element, and therefore, the implicit case. only specifically
mention ecoff_probe_func where necessary.
if we decide to not use this c99 feature for now, at least there's now
less stuff to rip out.