Add support for DWARF-4 line number tables.

Reads the maximum_operations_per_instruction field, added in DWARF-4 to the
line number program header.

gold/
	* PR gold/22776
	* dwarf_reader.cc (Sized_dwarf_line_info::read_header_prolog): Add
	support for DWARF-4 line number tables.
	* dwarf_reader.h (Sized_dwarf_line_info::max_ops_per_insn): New field.