Implement --push-state/--pop-state.

Implement --push-state/--pop-state.

gold/
	PR gold/18989
	* options.cc (General_options::object_format_to_string): New function.
	(General_options::copy_from_posdep_options): New function.
	(General_options::parse_push_state): New function.
	(General_options::parse_pop_state): New function.
	* options.h (--push-state, --pop-state): New options.
	(General_options::object_format_to_string): New method.
	(General_options::set_incremental_disposition): New method.
	(General_options::copy_from_posdep_options): New method.
	(General_options::options_stack_): New data member.