Fix DWARF reader to use correct size for DW_FORM_ref_addr.

2017-11-19  Ian Lance Taylor  <iant@google.com>
	    Cary Coutant  <ccoutant@gmail.com>

gold/
	* gold/dwarf_reader.h (class Dwarf_info_reader): Add ref_addr_size
	method.
	* gold/dwarf_reader.cc (Dwarf_die::read_attributes): Use ref_addr_size
	for DW_FORM_ref_addr_size.
	(Dwarf_die::skip_attributes): Likewise.