Fix .debug_rnglists generation with -gdwarf-5 -gsplit-dwarf.

Fix .debug_rnglists generation with -gdwarf-5 -gsplit-dwarf.

Early debug broke generation of .debug_rnglists when using both -gdwarf5
and -gsplit-dwarf. It introduces a generation for init_sections_and_labels,
but doesn't account for the generation of up to 4 unique ranges labels,
two created in init_sections_and_labels and two in output_rnglists.
Fix this by passing generation to output_rnglists and creating 4 unique
labels per generation.

Without this fix using -gdwarf-5 -gsplit-dwarf could result in:
  Error: symbol `.Ldebug_ranges2' is already defined

gcc/ChangeLog:

       * dwarf2out.c (init_sections_and_labels): Use generation to create
       unique ranges_section_label and ranges_base_label. Return generation.
       (output_rnglists): Add generation argument. Use generation to create
       unique ranges labels.
       (dwarf2out_finish): Get generation from init_sections_and_labels
       and pass generation to output_rnglists.

From-SVN: r255107