DWARF-5: Fix -fdebug-types-section

DWARF-5: Fix -fdebug-types-section

GDB was now accessing as signatured_type memory allocated only by size of
dwarf2_per_cu_data.

gdb/ChangeLog
2017-08-24  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* dwarf2read.c (build_type_psymtabs_reader): New prototype.
	(process_psymtab_comp_unit): Accept IS_DEBUG_TYPES.
	(read_comp_units_from_section): New parameter abbrev_section, use
	read_and_check_comp_unit_head, allocate signatured_type if needed.
	(create_all_comp_units): Update read_comp_units_from_section caller.