libdw: Add DW_MACRO constants and DW_MACRO_GNU compatibility defines.

libdw: Add DW_MACRO constants and DW_MACRO_GNU compatibility defines.

Accept version 5 .debug_macro format, which is identical to the GNU
version 4 format. No real support yet for the new supplementary object
file (sup) and indirect string references (strx).  GCC doesn't generate
them yet. readelf does recognize them, but doesn't try to decode them.
dwarf_getmacros currently rejects the new formats.

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