Add --orphan-handling option.

Add --orphan-handling option.

gold/
	PR gold/20749
	* options.h (--orphan-handling): New option.
	(General_options::Orphan_handling): New enum.
	(General_options::orphan_handling_enum): New method.
	(General_options::set_orphan_handling_enum): New method.
	(General_options::orphan_handling_enum_): New data member.
	* options.cc (General_options::General_options): Initialize new member.
	(General_options::finalize): Convert --orphan-handling argument to enum.
	* script-sections.cc (Script_sections::output_section_name): Check it.