Remove Atomic.h

The STL has everything we need nowadays.

I have tried to not alter any behavior or semantics with this
change wherever possible. In particular, WriteLow and WriteHigh
in CommandProcessor retain the ability to accidentally undo
another thread's write to the upper half or lower half
respectively. If that should be fixed, it should be done in a
separate commit for clarity. One thing did change: The places
where we were using += on a volatile variable (not an atomic
operation) are now using fetch_add (actually an atomic operation).

Tested with single core and dual core on x86-64 and AArch64.

Source/Core/VideoCommon/RenderBase.cpp

@@ -889,7 +889,9 @@ void Renderer::CheckFifoRecording()
     RecordVideoMemory();
   }
 
-  FifoRecorder::GetInstance().EndFrame(CommandProcessor::fifo.CPBase, CommandProcessor::fifo.CPEnd);
+  FifoRecorder::GetInstance().EndFrame(
+      CommandProcessor::fifo.CPBase.load(std::memory_order_relaxed),
+      CommandProcessor::fifo.CPEnd.load(std::memory_order_relaxed));
 }
 
 void Renderer::RecordVideoMemory()