GAS: Add a return type tag to DWARF DIEs generated for function symbols.

GAS: Add a return type tag to DWARF DIEs generated for function symbols.

	PR 29517
	* dwarf2dbg.c (GAS_ABBREV_COMP_UNIT): New defined constant.
	(GAS_ABBREV_SUBPROG): New defined constant.
	(GAS_ABBREV_NO_TYPE): New defined constant.
	(out_debug_abbrev): Use the new defined constants when emitting
	abbreviation numbers.  Generate an abbreviation for an unspecified
	type.
	(out_debug_info): Use the new defined constants when referring to
	abbreviations.  Generate a use of the no_type abbreviation.
	Reference the use when generating DIEs for functions.
	* testsuite/gas/elf/dwarf-3-func.d: Update to allow for newly
	extended output from the assembler.
	* testsuite/gas/elf/dwarf-5-func-global.d: Likewise.
	* testsuite/gas/elf/dwarf-5-func-local.d: Likewise.
	* testsuite/gas/elf/dwarf-5-func.d: Likewise.