libdw: dwarf_elf_begin should use either plain, dwo or lto DWARF sections.

libdw: dwarf_elf_begin should use either plain, dwo or lto DWARF sections.

When opening an ELF file that contained a mix of plain, dwo or lto .debug
sections the result could be confusing. Add a check to pick just the plain
.debug sections, or the .dwo sections or the .gnu.debuglto_.debug sections
(in that order of preference). That way there is always a consistent set.

https://sourceware.org/bugzilla/show_bug.cgi?id=27367

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