[C++] avoid compiler reordering statements.

std::atomic_signal_fence 

Equivalent to std::atomic_thread_fence, except no CPU instructions for memory ordering are issued.

Only reordering of the instructions by the compiler is suppressed as order instructs.

Before C++11, usually this is coded with same affect:

// Equivalent to a full compiler-only memory barrier (like memory_order_seq_cst)
__asm__ __volatile__("" ::: "memory");

or on M$ platform:

// Equivalent to a compiler-only memory barrier
_ReadWriteBarrier();