Teach the readelf about arm64 gnu properties

[andrew]

Summary

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.

[emaste]

contrib/elftoolchain/readelf/readelf.c

3550

PAV?

3633

We probably need to handle the NULL case?

[andrew]

Fix the spelling of PAC

[andrew]

contrib/elftoolchain/readelf/readelf.c

3633

The NULL case here is the same as the missing default case in the old version.

[emaste]

contrib/elftoolchain/readelf/readelf.c

3633

Ok, fair point. We ought to add it, I don't care if it's a later commit (or if I have to do it)