xtensa: set no optimization for arch_cpu_idle() with xt-clang

xt-clang likes to remove any consecutive NOPs more than 8. So
we need to force the function to have no optimization to avoid
this behavior and to retain all those NOPs.

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

arch/xtensa/core/cpu_idle.c

@@ -3,8 +3,19 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
+#include <zephyr/toolchain.h>
 #include <zephyr/tracing/tracing.h>
 
+/* xt-clang removes any NOPs more than 8. So we need to set
+ * no optimization to avoid those NOPs from being removed.
+ *
+ * This function is simply enough and full of hand written
+ * assembly that optimization is not really meaningful
+ * anyway. So we can skip optimization unconditionally.
+ * Re-evalulate its use and add #ifdef if this assumption
+ * is no longer valid.
+ */
+__no_optimization
 void arch_cpu_idle(void)
 {
 	sys_trace_idle();