add spawn thread API and sample (#333)

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

@@ -0,0 +1,15 @@
+/*
+ * Copyright (C) 2019 Intel Corporation.  All rights reserved.
+ * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+ */
+
+int sum(int start, int length)
+{
+    int sum = 0, i;
+
+    for (i = start; i < start + length; i++) {
+        sum += i;
+    }
+
+    return sum;
+}