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 bug42;

void main() {
    int i = 2;
    switch (i) {
    case 0:
    case 1:
    default:
    }
}