kldxref: Simplify handling of ELF object files

Unlike the backend for ELF DSOs, the object file backend allocated an
aligned chunk of memory and read all of the in-memory sections from
the file into this memory even though most of the file contents were
never used. Instead, just track a set of virtual addresses (based at
0) that each loaded section would be loaded at and only read the
necessary bits from the backing file when needed.

Reviewed by: imp
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D43126