tests: add handling for rx arch

empty define for this architecture use for testing.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>

Author: nashif

subsys/testsuite/include/zephyr/test_asm_inline_gcc.h

@@ -48,6 +48,8 @@ static inline void timestamp_serialize(void)
 #define timestamp_serialize()
 #elif defined(CONFIG_MIPS)
 #define timestamp_serialize()
+#elif defined(CONFIG_RX)
+#define timestamp_serialize()
 #else
 #error implementation of timestamp_serialize() not provided for your CPU target
 #endif