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)

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

void main()
{
    int[] arr = [1];
    foreach(s; arr)
    {
        void foo()
	{
	    printf("n %d\n", s);
	}

	printf("o1 %d\n", s);
	foo();
	printf("o2 %d\n", s);
    }
}