readelf: Use raw section data if nothing is available through libdw.

readelf: Use raw section data if nothing is available through libdw.

For various debug data sections readelf has its own parsers that don't
rely on libdw data structures or functions. But we still like to get the
data through libdw since that will be uncompressed and/or relocated.
But there can be reasons for libdw to have rejected the section data.
In that case we want to try to parse the "raw" section data.

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