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.