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)

template toInt(char[4] arg)
{
	const uint toInt = (cast(uint[]) arg)[0];
	}
	 
	 void main()
	 {
	   auto i = toInt!("abcd");
	   }