Adapt the _ALT style to our new standard

- in .h files: only put the context declaration inside the #ifdef _ALT
  (this was changed in 2.9.0, ie after the original PR)
- in .c file: only leave selftest out of _ALT: even though some function are
  trivial to build from other parts, alt implementors might want to go another
way about them (for efficiency or other reasons)

library/chacha20.c

@@ -354,8 +354,6 @@ int mbedtls_chacha20_update( mbedtls_chacha20_context *ctx,
     return( 0 );
 }
 
-#endif /* !MBEDTLS_CHACHA20_ALT */
-
 int mbedtls_chacha20_crypt( const unsigned char key[32],
                             const unsigned char nonce[12],
                             uint32_t counter,
@@ -383,6 +381,8 @@ cleanup:
     return( result );
 }
 
+#endif /* !MBEDTLS_CHACHA20_ALT */
+
 #if defined(MBEDTLS_SELF_TEST)
 
 static const unsigned char test_keys[2][32] =