SL-18837: Enlarge default coroutine stack size.

A test executable on a GitHub Windows runner failed with C00000FD, which
reports stack overflow.

(cherry picked from commit aab7b4ba3812e5876b1205285bcfd8cff96bcac9)

indra/llcommon/llcoros.cpp

@@ -123,11 +123,7 @@ LLCoros::LLCoros():
     // Previously we used
     // boost::context::guarded_stack_allocator::default_stacksize();
     // empirically this is insufficient.
-#if ADDRESS_SIZE == 64
-    mStackSize(512*1024),
-#else
-    mStackSize(256*1024),
-#endif
+    mStackSize(768*1024),
     // mCurrent does NOT own the current CoroData instance -- it simply
     // points to it. So initialize it with a no-op deleter.
     mCurrent{ [](CoroData*){} }