[gdb/testsuite] Support .debug_line v5 in dwarf assembler

[gdb/testsuite] Support .debug_line v5 in dwarf assembler

The v5 section version for .debug_line has:
- two new fields address_size and segment_selector_size
- a different way to encode the directory and filename tables.

Add support for this in the dwarf assembler.

For now, make the v5 directory and filename tables work with the v4 type of
specification in the test-cases by adding duplicate entries at position 0.

This will need to be properly fixed with an intrusive fix that changes how
directory and filename entries are specified in the test-cases, f.i:
...
set diridx [include_dir "${srcdir}/${subdir}"]
set fileidx [file_name "$srcfile" $diridx]
...

Tested on x86_64-linux.