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 mini.norun_debug2;

import tango.stdc.stdlib : rand;

void main()
{
    size_t iter;
    while (iter < 25)
    {
        if (rand() % 20 == 10)
            *cast(int*)null = 0;
        ++iter;
    }
    assert(0);
}