[compiler-rt] [libcxxabi] [libunwind] [runtimes][PAC] Harden unwinding when possible (#138571) (PR #143230)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

I'll be back to working on this later today - there's a pile of feedback, and a bunch of nonsense that I managed to accidentally accrue and/or re-format when pushing the PR so it will take a little time to unbreak that. It may be easiest to force push a repaired PR first and then address comments.

I also realize that some of the breakage might be due to mismatched __APPLE__ vs has_feature(actual_feature)

https://github.com/llvm/llvm-project/pull/143230