gas: Clear all auto-assigned file slots

Since a file slot is auto-assigned for the #APP marker appeared before
the first .file <NUMBER> directive has been seen, clear all auto-assigned
file slots when seeing the first .file <NUMBER> directive.

	PR gas/26778
	* * dwarf2dbg.c (num_of_auto_assigned): New.
	(allocate_filenum): Increment num_of_auto_assigned.
	(dwarf2_directive_filename): Clear the slots auto-assigned
	before the first .file <NUMBER> directive was seen.
	* testsuite/gas/i386/dwarf4-line-1.d: New file.
	* testsuite/gas/i386/dwarf4-line-1.s: Likewise.
	* testsuite/gas/i386/i386.exp: Run dwarf4-line-1.