Support: reduce stack used in default size test.

When the sanitizers aren't enabled they can use more than 1KB of stack, causing
an overflow where there shouldn't be.

Should fix Green Dragon test.

unittests/Support/Threading.cpp

@@ -82,7 +82,7 @@ TEST(Threading, RunOnThreadSync) {
 #if defined(__APPLE__)
 TEST(Threading, AppleStackSize) {
   llvm::thread Thread([] {
-    volatile unsigned char Var[8 * 1024 * 1024 - 1024];
+    volatile unsigned char Var[8 * 1024 * 1024 - 10240];
     Var[0] = 0xff;
     ASSERT_EQ(Var[0], 0xff);
   });