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/CommandProcessor.h

@@ -4,6 +4,8 @@
 
 #pragma once
 
+#include <atomic>
+
 #include "Common/CommonTypes.h"
 
 class PointerWrap;
@@ -17,15 +19,15 @@ namespace CommandProcessor
 struct SCPFifoStruct
 {
   // fifo registers
-  volatile u32 CPBase;
-  volatile u32 CPEnd;
+  std::atomic<u32> CPBase;
+  std::atomic<u32> CPEnd;
   u32 CPHiWatermark;
   u32 CPLoWatermark;
-  volatile u32 CPReadWriteDistance;
-  volatile u32 CPWritePointer;
-  volatile u32 CPReadPointer;
-  volatile u32 CPBreakpoint;
-  volatile u32 SafeCPReadPointer;
+  std::atomic<u32> CPReadWriteDistance;
+  std::atomic<u32> CPWritePointer;
+  std::atomic<u32> CPReadPointer;
+  std::atomic<u32> CPBreakpoint;
+  std::atomic<u32> SafeCPReadPointer;
 
   volatile u32 bFF_GPLinkEnable;
   volatile u32 bFF_GPReadEnable;