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 globals1;
extern(C) int printf(char*, ...);

char[] gstr = "hello world";

void main()
{
    printf("%.*s\n", gstr.length, gstr.ptr);
    char[] str = gstr;
    printf("%.*s\n", str.length, str.ptr);
}