Disable aux stack allocations for threads spawned by wasi_thread_start (#1867)

This syscall doesn't need allocating stack or TLS and it's expected from the application
to do that instead. E.g. WASI-libc already does this for `pthread_create`.

Also fix some of the examples to allocate memory for stack and not use stack before
the stack pointer is set to a correct value.

samples/wasi-threads/wasm-apps/wasi_thread_start.h

@@ -0,0 +1,32 @@
+/*
+ * Copyright (C) 2022 Amazon.com Inc. or its affiliates. All rights reserved.
+ * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+ */
+#ifndef WASI_THREAD_START_H
+#define WASI_THREAD_START_H
+
+#define STACK_SIZE 1024
+
+typedef struct {
+    void *stack;
+} start_args_t;
+
+static inline int
+start_args_init(start_args_t *start_args)
+{
+    start_args->stack = malloc(STACK_SIZE);
+    if (!start_args->stack) {
+        return 0;
+    }
+
+    start_args->stack += STACK_SIZE;
+    return 1;
+}
+
+static inline void
+start_args_deinit(start_args_t *start_args)
+{
+    free(start_args->stack - STACK_SIZE);
+}
+
+#endif