gdb/dwarf: remove line_header::total_length field

This doesn' have to be a field, it can simply be a local variable in
dwarf_decode_line_header.  Name the local variable "unit_length", since
that's what the field in called in DWARF 4 and 5.  It's always easier to
follow the code with the standard on the side when we use the same
terminology.

Change-Id: I3ad1022afd9410b193ea11b9b5437686c1e4e633