Add support for --dwarf-[3|4|5] to assembler command line.

Add support for --dwarf-[3|4|5] to assembler command line.

	PR 25612
	* as.c (dwarf_level): Define.
	(show_usage): Add --gdwarf-3, --gdwarf-4 and --gdwarf-5.
	(parse_args): Add support for the new options.
	as.h (dwarf_level): Prototype.
	* dwarf2dbg.c (DWARF2_VERSION): Use dwarf_level as default version
	value.
	* config/tc-ia64.h (DWARF2_VERISION): Update definition.
	(DWARF2_LINE_VERSION): Remove definition.
	* doc/as.texi: Document the new options.