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

[Oliver Hunt via llvm-commits]

ojhunt wrote:

Force push+rebase because I've significantly reworked and tidied the changes.

I've centralized the schema definitions in each library, and made them explicit rather than constructed at each site.

I've also replaced the __APPLE__ guards with feature guards which seems much more reasonable - the places I thought __APPLE__ made sense we really intra library PAC which makes that kind of silly.

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