gas: Reuse the input file entry in the file table

Some instructions can be emitted (dwarf2_emit_insn is called) before the
first .file <NUMBER> directive has been seen, which allocates the input
file as the first file entry.  Reuse the input file entry in the file
table.

	PR gas/25878
	PR gas/26740
	* dwarf2dbg.c (file_entry): Remove auto_assigned.
	(assign_file_to_slot): Remove the auto_assign argument.
	(allocate_filenum): Updated.
	(allocate_filename_to_slot): Reuse the input file entry in the
	file table.
	(dwarf2_where): Replace as_where with as_where_physical.
	* testsuite/gas/i386/dwarf5-line-1.d: New file.
	* testsuite/gas/i386/dwarf5-line-1.s: Likewise.
	* testsuite/gas/i386/i386.exp: Run dwarf5-line-1.

(cherry picked from commit 6915020b)

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.

(cherry picked from commit bd0c565e)

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.

(cherry picked from commit ae9d2233)