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.S

@@ -0,0 +1,22 @@
+# A slightly modified copy of the wasi-libc implementation
+# https://github.com/WebAssembly/wasi-libc/pull/376/
+	.globaltype	__stack_pointer, i32
+	.functype	__wasi_thread_start_C (i32, i32) -> ()
+
+	.globl	wasi_thread_start
+
+wasi_thread_start:
+	.functype	wasi_thread_start (i32, i32) -> ()
+
+	# Set up the minimum C environment.
+	# Note: offsetof(start_arg, stack) == 0
+	local.get   1  # start_arg
+	i32.load    0  # stack
+	global.set  __stack_pointer
+
+	# Make the C function do the rest of work.
+	local.get   0  # tid
+	local.get   1  # start_arg
+	call __wasi_thread_start_C
+
+	end_function