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)

struct S {
    int a; int b; int c; int d = 7;
}
void test(int i) {
    S s = { 1, i };   // q.a = 1, q.b = i, q.c = 0, q.d = 7
    assert(s.a == 1);
    assert(s.b == i);
    assert(s.c == 0); // line 8
    assert(s.d == 7);
}
void main() {
    test(42);
}