Fix cfloat return on x86_64: only perform ABI transformation for non-extern(D) functions. There's no need to waste cycles with extern(D), which we get to define ourselves. Fixes tests/mini/asm8.d. (Since the asm abiret code already assumed {xmm0, xmm1} returns)

module virtcall;

extern(C) int printf(char*, ...);

class C
{
    override char[] toString()
    {
        return "overridden";
    }
}

void main()
{
    C c = new C;
    auto s = c.toString();
    printf("%.*s\n", s.length, s.ptr);
}