[objcopy] Fix quadratic-time when handling --redefine-syms

[objcopy] Fix quadratic-time when handling --redefine-syms

objcopy/
	* objcopy.c (struct redefine_node): Delete the field "next".
	(redefine_sym_list): Deleted.
	(redefine_specific_htab): New hash table.
	(redefine_specific_reverse_htab): Likewise.
	(eq_string_redefnode): New function.
	(htab_hash_redefnode): Likewise.
	(create_symbol2redef_htab): Likewise.
	(add_specific_symbol_node): Likewise.
	(create_symbol_htabs): Create redefine_specific_htab and
	redefine_specific_reverse_htab.
	(lookup_sym_redefinition): Use hash table instead of list.
	(redefine_list_append): Likewise, and rename to add_redefine_and_check.
	(copy_main): Use redefine_specific_htab instead of redefine_sym_list.
	Update comments.