atomic.hpp: add some features and optimizations

Add atomic_t<>::observe() (relaxed load)
Add atomic_fence_XXX() (barrier functions)
Get rid of MFENCE instruction, replace with no-op LOCK OR on stack.
Remove <atomic> dependence from stdafx.h and relevant headers.

rpcs3/Emu/Cell/RawSPUThread.cpp

@@ -105,7 +105,7 @@ bool spu_thread::read_reg(const u32 addr, u32& value)
 		case MFC_EIEIO_CMD:
 		case MFC_SYNC_CMD:
 		{
-			std::atomic_thread_fence(std::memory_order_seq_cst);
+			atomic_fence_seq_cst();
 			value = MFC_PPU_DMA_CMD_ENQUEUE_SUCCESSFUL;
 			return true;
 		}