Support ZLIB compressed debug sections

Support ZLIB compressed debug sections

objcopy --compress-debug-sections uses ZLIB compression to reduce debug
sections, which can sometimes be larger than the size of the binary
itself. This change makes pyelftools consider compressed debug sections
when checking for DWARF data.

readelf tool supports other types of compressed sections
(https://github.com/facebook/binutils/blob/master/binutils/readelf.c#L12038)
but their support is outside of scope of this change.

Test plan:
  $ ./test/run_all_unittests.py

Signed-off-by: Stanislas P1kachu Lejay <p1kachu@lse.epita.fr>