gold: Skip address size and segment selector for DWARF5

The .debug_line secton in DWARF5 has a byte for address size and a byte
for segment selector after DWARF version.  Skip them for DWARF5.

	PR gold/27246
	* dwarf_reader.cc (Sized_dwarf_line_info::read_header_prolog):
	Skip address size and segment selector for DWARF5.

(cherry picked from commit 0318cca4)