libdwfl: Use elf_compress[_gnu] to decompress string, symbol and reloc data.

This makes usage of the libdwfl symbol functions work out of the box even
when some sections (string, symbol or xndx) are compressed. For ET_REL
files this makes relocations just work by making sure the target section
is decompressed first before relocations are applied.

Signed-off-by: Mark Wielaard <mjw@redhat.com>