small fix for 64-Bit trace

[MartinNowak]

Pretty obvious fix but I still have a small regression test.
I Just don't know where to put it?

---

struct FourUShort {
this(ushort a, ushort b, ushort c, ushort d) {
this.a = a;
this.b = b;
this.c = c;
this.d = d;
}
ushort a, b, c, d;
}

void main()
{
auto f = FourUShort(0, 1, 2, 3);
assert(f.a == 0);
assert(f.b == 1);
assert(f.c == 2);
assert(f.d == 3);
}

[braddr]

I'll add this to the dmd test suite. It's well equipped to run this sort of test and I don't think it's terribly egregious to use it to test low level druntime pieces through it.