as: Automatically enable DWARF5 support

as: Automatically enable DWARF5 support

Currently

$ as -o x.o x.s

fails when x.s contains DWARF5 ".file 0" or ".loc 0" directives.  Update
assembler to automatically enable DWARF5 support so that

$ gcc -S -g -c x.c
$ gcc -c x.s

works.

	PR gas/27195
	* dwarf2dbg.c (dwarf2_gen_line_info): Set dwarf_level to 5 if
	needed.
	(dwarf2_directive_filename): Likewise.
	(dwarf2_directive_loc): Likewise.
	* testsuite/gas/elf/dwarf-5-file0.d: Pass --gdwarf-3.
	* testsuite/gas/lns/lns-diag-1.l: Remove the
	"Error: file number less than one" errors.