fix Issue 21919 - darwin: SEGV in core.thread tests on OSX 11

Author: ibuclaw

src/core/thread/fiber.d

@@ -545,6 +545,16 @@ class Fiber
         // the existence of debug symbols and other conditions. Avoid causing
         // stack overflows by defaulting to a larger stack size
         enum defaultStackPages = 8;
+    else version (OSX)
+    {
+        version (X86_64)
+            // libunwind on macOS 11 now requires more stack space than 16k, so
+            // default to a larger stack size. This is only applied to X86 as
+            // the PAGESIZE is still 4k, however on AArch64 it is 16k.
+            enum defaultStackPages = 8;
+        else
+            enum defaultStackPages = 4;
+    }
     else
         enum defaultStackPages = 4;