gas: Always use as_where for preprocessed assembly codes

gas: Always use as_where for preprocessed assembly codes

Always clear the slot 1 if it was assigned to the input file before the
first .file <NUMBER> directive has been seen.  Always use as_where to
generate the correct debug infor for preprocessed assembly codes.

	PR gas/25878
	PR gas/26740
	* dwarf2dbg.c (allocate_filename_to_slot): Don't reuse the slot 1
	here.
	(dwarf2_where): Restore as_where.
	(dwarf2_directive_filename): Clear the slot 1 if it was assigned
	to the input file.
	* testsuite/gas/i386/dwarf5-line-2.d: New file.
	* testsuite/gas/i386/dwarf5-line-2.s: Likewise.
	* testsuite/gas/i386/dwarf5-line-3.d: Likewise.
	* testsuite/gas/i386/dwarf5-line-3.s: Likewise.
	* testsuite/gas/i386/i386.exp: Run dwarf5-line-2 and
	dwarf5-line-3.