tests/kernel/fatal: add volatile to prevent compiler optimization

Initialization of local variable 'illegal' can't be optimized, or the
program will jump to the memory contains random value which causes the
unexpected behavior. Add volatile to local variable 'illegal' to prevent
compiler optimization.

Signed-off-by: Jim Shu <cwshu@andestech.com>

tests/kernel/fatal/src/main.c

@@ -82,7 +82,7 @@ void alt_thread1(void)
 	 * and xtensa
 	 */
 	{
-		long illegal = 0;
+		volatile long illegal = 0;
 		((void(*)(void))&illegal)();
 	}
 #endif