[Issue 21919] darwin: SEGV in core.thread tests on OSX 11

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21919

--- Comment #9 from Iain Buclaw <ibuclaw at gdcproject.org> ---
Done some prodding around, and the root cause is darwin's libunwind now
overflows the Fiber's small 16kb stack.

Fix then is to bump the stack allocated for Fibers.

     version (Windows)
         // exception handling walks the stack, invoking DbgHelp.dll which
         // needs up to 16k of stack space depending on the version of
DbgHelp.dll,
         // 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)
+            enum defaultStackPages = 8;
+        else
+            enum defaultStackPages = 4;
+    }
     else
         enum defaultStackPages = 4;

Darwin x86 pagesize is 4k, whilst arm64 is 16k, so this fix should only be
applied to 64-bit code.

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