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

[Sander de Smalen via cfe-commits]

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@@ -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.
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sdesmalen-arm wrote:

Some discrepancies with the compiler-rt version, is that this one does not have the `.variant_pcs` directive and BTI hint. Do those need adding?

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