Teach the readelf about arm64 gnu properties

On arm64 binaries can be tagged as using BTI or PAC. Add support to
decode these to the elftoolchain readelf.

To simplify the code use a table based method to find the flag
description table.

Reviewed by: emaste
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D32315