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 scope2;
extern(C) int printf(char*, ...);
void main()
{
    scope(exit) printf("exit\n");
    scope(failure) printf("failure\n");
    scope(success) printf("success\n");
}