libdw, readelf: Handle .debug_*.dwo section name variants.
The .debug_*.dwo section names are handled just like their none .dwo
variants. The section contents is the same as sections without the .dwo
name, but they are only found in split-dwarf files. This patch allows
opening and inspecting split-dwarf files. It doesn't yet connect the
split-dwarf with their skeleton (or the other way around). It also
doesn't yet handle any special split-dwarf attributes or tags.
Signed-off-by: Mark Wielaard <mark@klomp.org>