libdw: Recognize .debug_[ct]u_index sections in dwarf_elf_begin

libdw: Recognize .debug_[ct]u_index sections in dwarf_elf_begin

DWARF package (.dwp) files have a .debug_cu_index section and,
optionally, a .debug_tu_index section.  Add them to the list of DWARF
sections.

Unfortunately, it's not that simple: the other debug sections in a dwp
file have names ending with .dwo, which confuses the checks introduced
by commit 5b21e70216b8 ("libdw: dwarf_elf_begin should use either plain,
dwo or lto DWARF sections.").  So, we also have to special case
.debug_cu_index and .debug_tu_index in scn_dwarf_type and check_section
to treat them as TYPE_DWO sections.

Signed-off-by: Omar Sandoval <osandov@fb.com>