libdw: Add DWARF5 attributes.

libdw: Add DWARF5 attributes.

Add new DWARF5 attribute constant names to the attributes enum.
Also add reserved comments between non-consecutive (reserved) numbers.

Remove DW_AT_subscr_data, DW_AT_element_list and DW_AT_member from the
enum list and turn them into compatibility defines because they are
not part of DWARF2+. That way code that directly references them still
compiles but they won't show up in known-dwarf.h.

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