This adds the machinery for cross reference points (hence "xref") for
things to be annotated with source code location or other metadata
and/or to be uniquely identified and found at runtime or by dissecting
executable files.
The extraction tool to walk down an ELF file is done and working but
needs some more cleanup and will be added in a separate commit.
Signed-off-by: David Lamparter <equinox@diac24.net>