libdwarf: Support consumption of compressed ELF sections

Automatically decompress zlib-compressed debug sections when loading
them. This lets ctfcovert work on userland code after commit
c910570e7573 ("Use compressed debug in standalone userland debug files
by default").

Reported by: avg
Reviewed by: avg, emaste
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33139