Add support for a ".file 0" directive if supporting DWARF 5 or higher.

Add support for a ".file 0" directive if supporting DWARF 5 or higher.

	PR 25614
	* dwarf2dbg.c (dwarf2_directive_filename): Allow a file number of
	0 if the dwarf_level is 5 or more.  Complain if a filename follows
	a file 0.
	* testsuite/gas/elf/dwarf-5-file0.s: New test.
	* testsuite/gas/elf/dwarf-5-file0.d: New test driver.
	* testsuite/gas/elf/elf.exp: Run the new test.

	PR 25612
	* config/tc-ia64.h (DWARF2_VERISION): Fix typo.
	* doc/as.texi: Fix another typo.