gas/ecoff.c: don't use zero struct copies to init

It might have made sense once upon a time, but doesn't nowadays when
compilers expand memset inline.

	* ecoff.c (add_aux_sym_tir, allocate_scope, allocate_vlinks),
	(allocate_shash, allocate_thash, allocate_tag, allocate_forward),
	(allocate_thead, allocate_lineno_list): Use memset rather than
	copying zero struct.