gas: Use the directory name in .file 0

gas: Use the directory name in .file 0

DWARF5 allows .file 0 to take an optional directory name.  Set the entry
0 of the directory table to the directory name in .file 0.

	PR gas/28266
	* dwarf2dbg.c (get_directory_table_entry): Add an argument for
	the directory name in .file 0 and use it, instead of PWD.
	(allocate_filenum): Pass NULL to get_directory_table_entry.
	(allocate_filename_to_slot): Pass the incoming dirname to
	get_directory_table_entry.
	* testsuite/gas/elf/dwarf-5-file0-2.d: New file.
	* testsuite/gas/elf/dwarf-5-file0-2.s: Likewise.
	* testsuite/gas/elf/elf.exp: Run dwarf-5-file0-2.