wrap RBIMPL_ASSERT_OR_ASSUME in an ifdef as it isn't available on Ruby 2.7

Author: samyron

ext/json/ext/generator/generator.c

@@ -304,7 +304,10 @@ ALWAYS_INLINE(static) char *copy_remaining_bytes(search_state *search, unsigned
     // Optimistically copy the remaining 'len' characters to the output FBuffer. If there are no characters
     // to escape, then everything ends up in the correct spot. Otherwise it was convenient temporary storage.
 #if defined(__has_builtin) && __has_builtin(__builtin_memcpy)
+
+#ifdef RBIMPL_ASSERT_OR_ASSUME
     RBIMPL_ASSERT_OR_ASSUME(len < 16);
+#endif
 
     if (vec_len == 16 && len >= 4) {
         // If __builtin_memcpy is available, use it to copy between SIMD_MINIMUM_THRESHOLD and vec_len-1 bytes.