libdwarf: Fix error handling in _dwarf_elf_init()

We were not setting "ret" before jumping to the error path, so the
function returned success even when it had failed.

Reviewed by: emaste
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33420

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

libdwarf: Hide SHT_NOBITS sections.

gnu_debuglink external debug files will contain an .eh_frame section of
type SHT_NOBITS. libdwarf does not handle such sections (or rather, it
expects all debug sections to not have type SHT_NOBITS). Avoid loading
SHT_NOBITS sections, to be consistent with SGI libdwarf's handling of
this case.

PR: 239516
Diagnosed by: Paco Pascal <me@pacopascal.com>
Reviewed by: emaste (previous version)
Event: July 2020 Bugathon
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D25626

libdwarf: fix SHT_REL relocation processing

Relocation of type SHT_REL must use the current value as addend.

PR: 204084
Obtained from: NetBSD libdwarf_elf_init.c v1.4

libdwarf: Handle .rel relocations

Some architectures use .rel relocations (for debug data), so they must
be handled.

This was discovered from ctfconvert on ARM object files. The lack of
relocation handling caused all string lookups to return the string at
offset 0 in .debug_str, typically "FreeBSD clang version ..."

Reviewed by: gnn, imp, rpaulo (earlier version)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D1819

libdwarf: Add symbol value when processing .rela relocations

Reviewed by: kib, rpaulo
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D1826