[Toybox] [PATCH] readelf: fix -n for x86-64 ibt/shstk notes.

[enh]

I couldn't work out how to get gcc to actually produce such a thing, but
/bin/dbxtool on my debian box right now has them.

The big mistake here is that GNU property notes' data is always 8-byte
aligned, so we needed to skip that. That lets us get rid of the existing
loop termination hack to skip padding.

While I'm here -- since the symptom was running off the end of the file --
I've also added a bounds check in the property dumping loop.

I wish I had fuzzing infrastructure to run AFL++ against this every time
it changes... In lieu of that I do wonder whether we should add `readelf
-aW /bin/* > /dev/null` as a smoke test that "at least it works for all
the _valid_ binaries on the system you're testing on". That would have
caught this sooner.
---
 toys/other/readelf.c | 26 ++++++++++++++++----------
 1 file changed, 16 insertions(+), 10 deletions(-)
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