[libunwind] [libunwind][AArch64] Disable ZA before resuming from unwinding (on Linux) (PR #165451)

[Benjamin Maxwell via cfe-commits]

================
@@ -827,6 +827,54 @@ DEFINE_LIBUNWIND_FUNCTION(__unw_getcontext)
   ret
 #endif
 
+//
+// extern "C" bool __libunwind_Registers_arm64_za_disable()
+//
+// On return:
+//  success (true/false) is returned in x0
+//
+  .p2align 2
+DEFINE_LIBUNWIND_FUNCTION(__libunwind_Registers_arm64_za_disable)
+  // If TPIDR2_EL0 is null, the subroutine just disables ZA.
----------------
MacDue wrote:

> I believe the two have different purposes, where `bti` protects the entry point into the function and `pac` authenticates the return address.

I was referring to `pacibsp` acting as an implicit `bti` see:
https://developer.arm.com/documentation/ddi0602/2025-09/Base-Instructions/PACIB--PACIB1716--PACIBSP--PACIBZ--PACIZB--Pointer-Authentication-Code-for-instruction-address--using-key-B-

However, I've also just noticed:
```
#define DEFINE_LIBUNWIND_FUNCTION(name)                                        \
  .globl SYMBOL_NAME(name) SEPARATOR                                           \
  HIDDEN_SYMBOL(SYMBOL_NAME(name)) SEPARATOR                                   \
  SYMBOL_IS_FUNC(SYMBOL_NAME(name)) SEPARATOR                                  \
  PPC64_OPD1                                                                   \
  SYMBOL_NAME(name):                                                           \
  PPC64_OPD2                                                                   \
  AARCH64_BTI
#endif
```

So the `DEFINE_LIBUNWIND_FUNCTION` will also add a `bti`.

---

> The function preserves more registers than the normal calling convention. If lazy-binding is enabled then the resolver function that finds the address, updates the GOT and calls this function should also preserve the same registers as the function that is being called. Otherwise, the first call to this function does not preserve the same registers.

I've added `.variant_pcs` for completeness, though I don't believe lazy binding will be used here (I think this is a local symbol statically linked into libunwind).  

https://github.com/llvm/llvm-project/pull/165451