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/aesni.c

@@ -39,6 +39,7 @@
 #include <immintrin.h>
 #endif
 
+#if !defined(MBEDTLS_AES_HAS_NO_BUILTIN)
 /*
  * AES-NI support detection routine
  */
@@ -68,6 +69,7 @@ int mbedtls_aesni_has_support(unsigned int what)
 
     return (c & what) != 0;
 }
+#endif /* !MBEDTLS_AES_HAS_NO_BUILTIN */
 
 #if MBEDTLS_AESNI_HAVE_CODE == 2