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.