don't emit debug sections until they are required

don't emit debug sections until they are required

gcc:

	* dwarf2out.c (macinfo_entry): New struct.
	(output_comp_unit): Emit the section start label here and flag that we
	will emit an info section.
	(dwarf2out_start_source_file): Save data in a macinfo entry rather than
	emitting directly.
	(dwarf2out_end_source_file): Likewise.
	(dwarf2out_define): Likewise.
	(dwarf2out_undef): Likewise.
	(output_macinfo): New.
	(dwarf2out_init): Do not emit debug section switches here, allocate a
	vec for macinfo, when required.
	(dwarf2out_finish): First switch to debug_abbrev_section here.
	debug_line_section,  debug_macinfo_section, Likewise.
	Check that the pubtypes table has at least one unpruned entry before
	trying to emit it.

From-SVN: r166704