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