libdw: Handle split dwarf debuglines.

libdw: Handle split dwarf debuglines.

Split DWARF .dwo files do contain a .debug_line section, but only with
the file table, there is no actual line program. Also split DWARF CU DIEs
don't have a DW_AT_stmt_list attribute. To get at the file (and dir) table
for a split unit DIE take just the files from the .debug_line section
(at offset zero). To get the full line table use the skeleton DIE (which
does have a DW_AT_stmt_list).

Signed-off-by: Mark Wielaard <mark@klomp.org>