xtensa: xcc: add a dummy atexit()

Some XCC toolchains do not provide atexit() which results
in undefined reference error. So add a weak dummy atexit()
for this siutation.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>

arch/xtensa/core/CMakeLists.txt

@@ -22,6 +22,10 @@ zephyr_library_sources_ifdef(CONFIG_DEBUG_COREDUMP coredump.c)
 
 zephyr_library_sources_ifdef(CONFIG_GDBSTUB gdbstub.c)
 
+if("${ZEPHYR_TOOLCHAIN_VARIANT}" STREQUAL "xcc")
+  zephyr_library_sources(xcc_stubs.c)
+endif()
+
 zephyr_library_include_directories(include)
 
 add_subdirectory(startup)

arch/xtensa/core/xcc_stubs.c

@@ -0,0 +1,14 @@
+/*
+ * Copyright (c) 2022 Intel Corporation
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <toolchain.h>
+
+__weak int atexit(void (*function)(void))
+{
+	ARG_UNUSED(function);
+
+	return 0;
+}