debug: Make sure to output .file 0 when generating DWARF5.

debug: Make sure to output .file 0 when generating DWARF5.

When gas outputs DWARF5 .debug_line[_str] then we have to tell it the
comp_dir and main file name for the zero entry line table. Otherwise
gas has to guess at the CU compilation directory and file.

Before a gcc -gdwarf-5 ../src/hello.c line table looked like:

Directory table:
 0     ../src (24)
 1     ../src (24)
 2     /usr/include (31)

File name table:
 0     hello.c (16),  0
 1     hello.c (16),  1
 2     stdio.h (44),  2

With this patch it looks like:

Directory table:
 0     /tmp/obj (0)
 1     ../src (24)
 2     /usr/include (31)

File name table:
 0     ../src/hello.c (9),  0
 1     hello.c (16),  1
 2     stdio.h (44),  2

gcc/ChangeLog:

	* dwarf2out.c (dwarf2out_finish): Emit .file 0 entry when
	generating DWARF5 .debug_line table through gas.