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 switch2;
extern(C) int printf(char*, ...);

void main()
{
    int i = 2;
    switch(i)
    {
    case 0: assert(0);
    case 1,2: printf("hello world\n"); break;
    default: assert(0);
    }
}