Add option to disable built-in aes implementation.

For time being, there are only two aes implementations for known
architectures. I define runtime detection function as const when
built-in was disabled. In this case, compiler will remove dead
built-in code.

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>

library/aesce.c

@@ -99,6 +99,7 @@
 #include <sys/auxv.h>
 #endif
 
+#if !defined(MBEDTLS_AES_HAS_NO_BUILTIN)
 /*
  * AES instruction support detection routine
  */
@@ -113,6 +114,7 @@ int mbedtls_aesce_has_support(void)
     return 1;
 #endif
 }
+#endif
 
 /* Single round of AESCE encryption */
 #define AESCE_ENCRYPT_ROUND                   \