Fix ICE in output_rnglists, at dwarf2out.c:12294

In this testcase the compile unit consists of a single
text section with a single embedded DECL_IGNORED_P function.
So we have a kind of multi-range text section here.
To avoid an ICE in output_rnglists we need to make sure
that have_multiple_function_sections is set to true.
This is a regression from
e69ac020 ("Add line debug info for virtual thunks")

2021-05-12  Bernd Edlinger  <bernd.edlinger@hotmail.de>

	PR debug/100515
	* dwarf2out.c (dwarf2out_finish): Set
	have_multiple_function_sections with multi-range text_section.

	* gcc.dg/debug/dwarf2/pr100515.c: New testcase.