Rosetta uses software emulation for x87 floating point. That's slow, but in practice that doesn't matter much. Mac software never had a reason to use x87 FP, every Intel Mac had at least SSE3 support.
long double x87me(long double a, long double b) {
return a+b;
}
pushq %rbp
movq %rsp, %rbp
fldt 32(%rbp)
fldt 16(%rbp)
faddp %st(1)
popq %rbp
retq