[libunwind] [libunwind][Haiku] Improve support (PR #115462)

[Fangrui Song via cfe-commits]

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@@ -2917,7 +2980,8 @@ template <typename A, typename R> int UnwindCursor<A, R>::step(bool stage2) {
 
   // Use unwinding info to modify register set as if function returned.
   int result;
-#if defined(_LIBUNWIND_CHECK_LINUX_SIGRETURN)
+#if defined(_LIBUNWIND_CHECK_LINUX_SIGRETURN) ||                               \
+    (defined(_LIBUNWIND_TARGET_HAIKU) && defined(_LIBUNWIND_TARGET_X86_64))
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MaskRay wrote:

Why is `_LIBUNWIND_TARGET_X86_64` check needed?

https://github.com/llvm/llvm-project/pull/115462