Fix problems emitting MIPS .reginfo section.

Fix problems emitting MIPS .reginfo section.

gold/
	* mips.cc (Mips_relobj::Mips_relobj): Initialize
	has_reginfo_section_.
	(Mips_relobj::has_reginfo_section_): New data member.
	(Mips_relobj::has_reginfo_section): New method.
	(class Mips_output_section_reginfo): Change base class to
	Output_section_data, and set masks of the output .reginfo section
	in constructor.
	(Mips_output_section_reginfo::as_mips_output_section_reginfo):
	Remove.
	(Mips_output_section_reginfo::set_masks): Likewise.
	(Mips_output_section_reginfo::set_final_data_size): Likewise.
	(Mips_output_section_reginfo::do_print_to_mapfile): New method.
	(Target_mips::do_make_output_section): Remove.
	(Mips_relobj::do_read_symbols): Set has_reginfo_section_ to true
	if the object contains a .reginfo section.
	(Target_mips::do_finalize_sections): Create a .reginfo output
	section if needed.