kernel: dynamic: declare dynamic stubs when disabled

With some of the recent work to disable unnecessary system
calls, there is a scenario where `z_impl_k_thread_stack_free()`
is not defined and an undefined symbol error occurs.

Safety was very concerned that dynamic thread stack code might
touch other code that does not malloc, so add a separate file
for the stack alloc and free stubs.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>

include/zephyr/kernel.h

@@ -289,6 +289,7 @@ __syscall k_thread_stack_t *k_thread_stack_alloc(size_t size, int flags);
  * @retval 0 on success.
  * @retval -EBUSY if the thread stack is in use.
  * @retval -EINVAL if @p stack is invalid.
+ * @retval -ENOSYS if dynamic thread stack allocation is disabled
  *
  * @see CONFIG_DYNAMIC_THREAD
  */

kernel/CMakeLists.txt

@@ -123,11 +123,11 @@ target_sources_ifdef(
   userspace.c
   )
 
-target_sources_ifdef(
-  CONFIG_DYNAMIC_THREAD
-  kernel PRIVATE
-  dynamic.c
-  )
+if(${CONFIG_DYNAMIC_THREAD})
+  target_sources(kernel PRIVATE dynamic.c)
+else()
+  target_sources(kernel PRIVATE dynamic_disabled.c)
+endif()
 
 target_include_directories(kernel PRIVATE
   ${ZEPHYR_BASE}/kernel/include

kernel/dynamic_disabled.c

@@ -0,0 +1,25 @@
+/*
+ * Copyright (c) 2022, Meta
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <errno.h>
+
+#include <zephyr/kernel.h>
+#include <zephyr/kernel/thread_stack.h>
+
+k_thread_stack_t *z_impl_k_thread_stack_alloc(size_t size, int flags)
+{
+	ARG_UNUSED(size);
+	ARG_UNUSED(flags);
+
+	return NULL;
+}
+
+int z_impl_k_thread_stack_free(k_thread_stack_t *stack)
+{
+	ARG_UNUSED(stack);
+
+	return -ENOSYS;
+}