std.int128 not working with -betterC enabled

[Renato]

I wanted to write a small program using betterC that needs to use 
int128.

This simple program works without -betterC:

```d
module dasc;

import std.int128;
import core.stdc.stdio;

extern (C):

int main() {
   auto n = Int128(128, 128);
   printf("n=%lu%lu", n.data.hi, n.data.lo);
   return 42;
}
```

But with -betterC:

```
dmd -L-ld_classic -betterC -run dasc.d
Undefined symbols for architecture x86_64:
   "__D3std6int1286Int1286__ctorMFNaNbNcNiNfllZSQBpQBoQBk", 
referenced from:
       _main in dasc.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to 
see invocation)
Error: linker exited with status 1

Compilation exited abnormally with code 1 at Wed Dec 20 19:11:56
```

I don't see anywhere whether int128 is supposed to work on 
-betterC. Is it not supported?