samples/spawn-thread: Tweak to expose a bug (#2888)

this would expose the deadlock bug, which has been fixed by
https://github.com/bytecodealliance/wasm-micro-runtime/pull/2882

samples/spawn-thread/wasm-apps/sum.c

@@ -3,6 +3,17 @@
  * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
  */
 
+/*
+ * have something in bss so that llvm synthesizes
+ * wasm start function for this module.
+ */
+char *
+return_bss()
+{
+    static char bss[4096];
+    return bss;
+}
+
 int
 sum(int start, int length)
 {
@@ -13,4 +24,4 @@ sum(int start, int length)
     }
 
     return sum;
-}
+}