Line Table Header variations.
The experimental version has a distinct version
number of 0xf006 so cannot be confused with DWARF4 or 5.

DWversion
2,3,4,5 meaning the DWARF standards
x meaning gcc's experimental two level line table.

2345x  1. unit_length (4 or 12 bytes) 
2345x  2. version (2 bytes, for x is 0xF006)
5      2.1  address_size
5      2.2  segment_selector_size
2345x  3. header_length (4 or 8 bytes)
2345x  4. minimum_instruction_length (1 byte)
45  x  5. maximum_operations_per_instruction (1 byte)
2345x  6. default_is_stmt (1 byte)
2345x  7. line_base (1 byte)
2345x  8. line_range (1 byte)
2345x  9. opcode_base (1 byte)
2345x 10. standard_opcode_lengths (array of length opcode_base - 1)
234 x 11. include directories (for x just one NUL byte(empty list))
234 x 12. file_names (for x just one NUL byte(empty list))
x     13. fake_extended_op (5 bytes: 00 FF FF 7F 7F)
x     14. logicals_table_offset (4 or 8 bytes)
x     15. actuals_table_offset (4 or 8 bytes)
x5    16. directory_entry_format_count (1 byte)
x5    17. directory_entry_format (sequence of unsigned LEB128 pairs)
x5    18. directories_count (unsigned LEB128)
x5    19. directories (sequence of directory entries)
x5    20. file_name_entry_format_count (1 byte)
x5    21. file_name_entry_format (sequence of unsigned LEB128 pairs)
x5    22. file_names_count (unsigned LEB128)
x5    23. file_names (sequence of directory entries)
x     24. subprogram_entry_format_count (1 byte)
x     25. subprogram_entry_format (sequence of unsigned LEB128 pairs)
x     26. subprograms_count (unsigned LEB128)
x     27. subprograms (sequence of directory entries)