[PAC][libunwind] Investigate if compatibility with v8.2-a could be enhanced

[kovdan01]

#143230 introduces libunwind hardening which could be used with pauth-enabled ABIs (such as arm64e for Apple and pauthtest for Linux). Note that these pauth-enabled ABIs imply proper v8.3-a. Apart from these ABIs, we also have pac-ret, which is a non-ABI-breaking option which could be used to sign return addresses and which should be compatible with armv8.2-a.

Probably, some of the changes introduced in #143230 might be changed to a armv8.2-a-compatible form as a part of unifcation task #160110.

See original comment from the PR below.

---

By the way, if switching to ptrauth_returns here, we most probably want to retain compatibility with Armv8.2-a by replacing

#if __has_feature(ptrauth_calls)
  retab
#else
  ret    x30                    // jump to pc
#endif

with

#if __has_feature(ptrauth_returns)
  autibsp
#endif
  ret    x30                    // jump to pc

That is, simply insert autibsp into the existing code.

Originally posted by @atrosinenko in #143230 (comment)

[kovdan01]

Tagging @asl @ojhunt @atrosinenko

[ojhunt]

@ahmedbougacha for his thoughts