Do away with hash table line lookup in dwarf2dbg.c

Hash lookup is silly when we can attach the line table info directly
to sections instead.  Worse, hash lookup fails when we have multiple
sections with the same name.

gas/
	* dwarf2dbg.c (all_segs_hash): Delete.
	(get_line_subseg): Delete last_seg, last_subseg, last_line_subseg.
	Retrieve line_seg for section via seg_info.
	* subsegs.h (segment_info_typet): Add dwarf2_line_seg.
gas/testsuite/
	* gas/elf/group2.d, * gas/elf/group2.s: New test.
	* gas/elf/elf.exp: Run it.