Optimize set_syscall_retval for arm64

[trasz]

Summary

Optimize set_syscall_retval() for arm64 by predicting the return
value to be zero.

[emaste]

similar to amd64,

if (__predict_true(error == 0)) {
        frame->tf_rax = td->td_retval[0];
        frame->tf_rdx = td->td_retval[1];
        frame->tf_rflags &= ~PSL_C;
        return;
}

[emaste]

Maybe worth a comment about the microoptimization?

[mhorne]

See also, the analogous commit for riscv: rS367138.