[libunwind] [PAC][libunwind][AArch64] Keep LR signed when stored in context struct (PR #171717)

[Oliver Hunt via cfe-commits]

https://github.com/ojhunt commented:

I just did an initial, very high level, scan through this and have a few questions in comments.

Most of this is the pure assembly parts of unwinding which I'm not super comfortable reviewing. If you feel it's necessary I can spend some time with pen and paper going through it, but I'm probably much slower than other folk due to my paranoia of missing details.

My only significant concern is the conditional signing behavior. It's not the worst (you're choosing between schemas rather the auth vs no-auth, but it would be nice for there to be some kinds of strict guarantees around the use of the validity and system concerns of  these variables and schemas. Im theory the use  b key, and similar 

Maybe @ahmedbougacha has thoughts on this?

https://github.com/llvm/llvm-project/pull/171717